Okay...I realize this is probably going to come across as crazed and deranged, but...

Is there some secret flag on a character that determines how well it learns evade?

I had a thief not long ago that had evade at around 87% inside about 30 hours and 25 ranks. Almost all of that was from level gains. Given how many skills were available each time I ranked, the odds of me getting 4-5 evade improvements seems pretty small if the selection is truly random.

Subsequently I had another character who, after ~120 hours and ~45 ranks, had evade at 77%. I think neither of those improvements was from level gains, but I could be wrong about that.

Purely due to RNG?

>Is there some secret flag on a character that determines how
>well it learns evade?

Nope!

Mostly it's RNG, especially in the cases where most of your gains come from level improves. When we're not talking level gains, it obviously can come a bit down to where you level -- going someplace where you're fighting stuff that dirts, kicks, disarms, etc. will give you more chances to learn.