[RandallBrown]

why does only one cylinder bind at a time?

It seems to me like the lock shouldn't be able to turn at all until all of the cylinders are pressed to the right height.

[SquareWheel]

Due to imperfections in the lock. It only takes a tiny difference for that tension to not be applied evenly so you can focus on one pin at a time.

[hoov]

Manufacturing tolerances aren't 100% perfect.