That's what Studio Ace said. A "100%" seal means you can create and maintain a vacuum. Customs are not designed to do that.
A good fit means the custom will comfortably fill the ear, and provide a full sound, while blocking out about 25db of outside noise. That doesn't mean every sound!
I would imagine a paranoid perfectionist would be the most difficult customer to please, because they demand:
!00% vacuum ("It's not sucking my brains out! - In space no one can hear you scream!")
100% sound isolation ("Darn!, I can still the pneumatic drill when I stand next to the workman!")
100% flawless, blemish free product (Impossible for a custom)
100% communication with live minute by minute update ( "I emailed 5 minutes ago, and they still haven't answered.")
100% fast delivery ("I sent my impressions yesterday, Why haven't I got my customs???")
They are so consumed with these things, it turns into a hissy fit when their impossible expectations are not met.
It's a wonder if they ever get to listen to music, or are interested in music, or even know what live music sounds like!!!
p.s Sorry that turned into a rant, which was not directed at you, but a general observation of some people's impossible expectations.
That's not to downplay the importance of high expectations, though.
That's how the US got us to the moon many decades ago.
And it's also how FMT got me to reconsider what an ER4P/S can deliver that my Livewires can't.
...But I still love my Livewires
My Livewires block my ears, as the fit is good, but I don't have a constant vacuum, suction effect going on.
People who are just after a vacuum (100% seal) are going to have problems as ear canals expand and contract with the heat and cold.
My Livewires also block effectively, whereas my first set didn't. I figure it's par for the course with customs manufacturers that there'll be a reasonable percentage of refit issues. I'm just saddened by experiences such as FMT's, which have lengthened the process out so far that the 'less than ideal experience' is more prominent in our minds than the many quieter expereiences where people are happy fairly early on and settle off to listen to music.