I don't agree that a good fit can still result in a bad seal.
I find that my CIEM's fit slightly differently day to day.
I'm only talking very minor differences though.
Also, often when I first insert my CIEM's they feel fairly tight, but after walking for an hour or more they can be removed and re-inserted more easily ( this is independent of sweat btw, as I've sometimes used an 'ear lube like oto-ease right from the off ).
When I've not used the tightest of my CIEM's for a while, inserting them for the first time is slightly uncomfortable ( fine once in though ) but I feel my ear 'stretches' to accommodate the shell and it's OK after that.
I guess what I'm saying is that even though the fit may be comfortable, if the seal is bad then the fit by definition is also bad.
None of my CIEM's suffer from a bad seal that breaks.
I think the Sensaphonics website has a good guide to checking the quality of your CIEM seal
This is my understanding too. If someone orderd a prefit service and is saying that the shell sits very good/perfect (this also means that is sits tight in the ear canal) then he should have a good seal.
And of course our ear canals changes slightly during the day.