Some notes:
Two separate products, even the same model, will always vary to a small degree. No two drivers will be exactly the same. No two enclosures will be exactly the same. Any electrical components used will not have the exact same values. There will always be slight variations, both between the left and right drivers and between different sets of the product.
Second, how accurate is the dB measurement using the iPhone? Have you tested the same earphone more than once to measure any natural variation in the readings? For example, depending on the method you could very well see 2-3dB variation with great ease. You have to be very careful about how you test and how repeatable the process is, both in how you physically do the test (exact location and distancing to the mic) and how accurate the testing device is (iPhone variation).
A question:
For burn in, did you doing anything with volume variation?
I ask because my approach isn't a duration approach but rather a ramp up approach. The goal of burn in is to loosen up new parts and get them largely to a state they will be during long term use. To do this, you have to push the hardware (safely). You can listen to a driver quietly for 100 hours and not do much with it. You can spend an hour and continually ramp up the volume (under straining) and get most of the break in done very, very quickly. There are general approaches, and there are efficient approaches. Break in is not a myth. It's a natural break down of new materials to a broken in state. However, you can approach the process many ways. I've gotten a number of earphones that have been used by other members for a good number of hours that were never fully broken in. Expecting it to happen fully naturally or through certain approaches isn't exactly a great approach. You really do need to put the earphone through its paces (safely within its actual capabilities, i.e. listening for harshness, strain, tonal change and staying below that level). In many cases, it's useful to have a powerful amp to aid the process and fully operate the earphone. For example, I got a pair of IE8 earphones with 30 hours on it that was never fully broken in by the previous owner. I have a decently powerful amp, and it takes a lot of that power to actually bring the IE8 which is a pretty output capable earphone to its limits. During break in it had several limit points as it loosened up. I would scale up the output till I could distinguish stressing and then backed down slightly. I let it sit for an hour playing. I put them on again and reset the volume, ramping it up more until the new stress point. I did this I think 4 times before I hit a static point that didn't change after repeat checks. To show the level of scale, I had the volume around half to start before showing strain. This was after around 30 hours of use by the previous owner and maybe 10 hours of use by me before deciding I really needed to finish breaking them in. I ended up at a hair under 4/5 by the time I was done fully breaking in the earphone. After maybe about 40 hours of mixed listening, it was only partially done. It took me half a day to casually finish up the process. Because of this and past experience with car/home drivers, I really dislike the duration type break in with the idea that time alone does it. I'm a big fan of a ramp up process, of course done safely, although some people have a poor perception or awareness of stress/strain/harshness/tonal change cues indicating the mechanical limits of the driver. I feel the ramp up process is quite fast with little need for such long burn in times. There is some discussion about the effect on frequency response and end sound due to burn in process. There is some discussion about the idea that you will end up with a different sounding product if you burn it in through a different method. I have no intention to touch that concept because there is such a challenge to prove. It ignores natural inconsistency in the products and assumes the micro variations will outweigh other macro factors of the driver, electronic, and earphone design.
