I have no evidence for this beyond the anecdotal, but:
Something I’ve noticed is that, in games where you make your characters in some detail before you start playing them—games like D&D, among lots of others—there’s a real disconnect between the conceptual, on-paper you have beforehand, and the way that character actually plays at the table, in-game. Once you’ve got that character at the table and have realized that the image you had in your head isn’t going to work out, there’s an adjustment period.
This adjustment varies, but it’s usually about five sessions. For an RPG that you’re new to, this is also time to learn the ropes of the game; it’s when you learn a new setting or lore; it’s when you really start to get to know each other at the table.
None of this is really that surprising.
What is surprising, at least to me, is that this adjustment keeps occurring, even with veteran players. The transition period from “this character that exists in my head” to “this is how my character plays at the table” happens with almost every character I play and with every player & character I run for. The first sessions or two are the wake-up call, then the next two or three is sorting out the pertinent details of how things are actually going to be.
Why does this happen? Or, I mean, I understand why it happens in the beginning, but why does this wall exist, even for veteran players?
Furthermore, is there a way to help cut that time down? To reduce the friction of introducing a character from concept in your head to the table itself?