I've been able to compare the UIEM and CIEM versions of countless IEMs by the luxury of living in Singapore, and the concept of there being differences is very widely-known, and it's widely-acknowledged as well. I remember hearing differences on in-ears from Custom Art, Lime Ears, Vision Ears, Jomo Audio, etc. It's to the point where, if one of the shop staff has the CIEM version in their possession and they feel the difference is big enough, they'd usually encourage me to demo the IEM by force-fitting their unit instead. I remember a funny instance where the roles were reversed and the shop only had the universal demo of the FIBAE 3, while I already had the custom. They were pretty disappointed with how theirs sounded until they tried my custom, after which they sent the demo back to Piotr.
They each sounded different in their own way, and the variables that caused this are endless. The most prevalent is the one @64Audio mentioned with regards to insertion depth and the acoustic hiccups that brings. For example, I preferred the UE LIVE with the IEMs sat at the very end of my ear canal, rather than inserted as deep as possible, while the custom sounded somewhere in the middle. But, then, there's also the tips being used thrown in there too. For example, I found the Lime Ears Aether R demo most sounded like its custom with MandarinEs's Symbio tips. And, 64's own demos sounded most like their custom counterparts with their foam, TrueFidelity tips. I think this is important to note, because demo units - including 64's - are most often auditioned with silicone tips, because they're easier to clean and they're less likely to break apart after lots of demos.
Another thing to mention aside from how tips affect the IEM's output acoustically (i.e. insertion depth, how sound bounces against the tip, etc.) is how tips affect our ears' behaviour. For example, my experience with SpinFit tips is they tend to tense up my ears, which shrinks the bass response and make IEMs seem brighter. The same can be true with Final Audio's tips when paired with IEMs of a certain shape. Empire Ears' ODIN and VE's ELYSIUM are recent examples where this happened with me. And, on the other hand, you've got foam tips that are typically looser or more malleable, which let your ears relax and help that bass breathe better. Again, though, that's just my experience. Perhaps, someone else with different ears may hear different changes or no changes at all. By the same token, CIEMs can sometimes fit tighter or looser depending on the make, which can cause these sorts of changes as well. So, even if the output of the demo was made to match the custom (or compensated to do so in some way), there's then the added factor of the fit mucking things up as well, and the manufacturer can't guarantee that a universal will fit your ears the way they want to. That's what customs are for.
The way manufacturers deal with this varies. You've got manufacturers that keep the internals the same and try to make the fit of the shell as custom-like as possible. An example would be Hidition, who used to make demos with giant canals and have people demo them without tips. They felt tips changed how their demos sounded, and this method achieved the most custom-like tuning. But, they eventually changed to the more traditional method, probably because people weren't too keen on holding the demos in place in their ears throughout the duration of the audition. Then you have guys like Piotr from Custom Art, who tunes the universals differently until they sound close to what he's hearing on his custom unit. It seems like 64 have a similar method, though I don't know whether it's done by ear like Piotr, or whether it's based on measurements.
So, yeah, ultimately, you can't come up with a perfect solution, because everyone's ears are different. You just gotta do the best you can and hope it works out. I personally don't recall being disappointed by any CIEM I've bought and preferring the universal. It's probably because I hate the feeling of tips in my ears in general, and I'd simply take the custom no matter what. But, as always, your mileage will vary. @twister6, for example, has the opposite thing where his ears can only take universals.
What I've learned is that if you like how a universal demo sounds with a certain pair of tips, then it's best to just get that universal with those tips. That'd be even more ideal if you already preferred universals in the first place. Going custom, sound difference risk aside, brings with it far better isolation (to most), fit (to some) and visual customisability. But, as far as just sound goes, this would probably be your safest - not best, but safest - bet, rather than the "get the custom if you like the demo" line that a lot of dealers tend to preach.
Not sure about other brands, but 64 Audio customs seem consistently darker than their universals, if the measurements are anything to go by (and anecdotally the A12t is definitely darker than the U12t).
Based on how both graphs look, it looks like there's an almost calculated difference between the universal and the custom. It strongly suggests the differences were intentional. Now, whether or not that difference is "good" and whether or not 64's compensation worked is an entirely different story. There's, again, that question of a difference between the demo and the custom being perhaps (unintentionally) dishonest for potential buyers. But, again, based on what I've been told by dealers, and based on images 64 have shown of their prototype Fourté, I'm under the impression that the universals are supposed to be worn with the foam TrueFidelity tips, which would theoretically bring the treble down and match the custom. Perhaps, that's what's causing the discrepancy. Of course, then there are differences like the N8's bass, which has been widely documented. But, as far as the treble goes, that would be my theory; dunno whether it's right or wrong.
Everyone tries their best to do so, I'd like to think. They each just do it differently. Piotr matches his demos to the custom by ear. Some may say it's a great method because it's using a human ear (and those of the IEM's designer, no less) as a reference, rather than a mic and a graph, while others may feel the opposite way. Again, there's no perfect method; just the ones that are as close to it as possible.
Unique Melody used to do this when I got my customs from them. Back when they reshelled IEMs, they'd also print out a graph comparing the original CIEM's response vs. the reshelled one's. I'm aware companies who do reshells like Ambient Acoustics do this as well. I'm championing for more manufacturers to do it too. At the same time, I've heard from some that they'd like to keep their "recipes" secret, so they're more reluctant to do this. It all depends on the brand, I think.
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