When you thumb-cock the gun, you probably do it pretty quickly, when you squeeze the DA trigger, you probably do it pretty slowly. When you turn the cylinder fast, once it gets going, the momentum will carry it to lock even if the hand comes out of engagement with the ratchet before lock is achieved. If the cylinder turns slowly, then the hand must stay in engagement all the way until the cylinder locks since there's no appreciable momentum to keep it going once the hand comes out of engagement with the ratchet. No matter how slowly the trigger is pulled, or the hammer is cocked, the cylinder should turn all the way to lock if everything is working properly.
Because of the way the hand engages the ratchet, if the ratchet can move forward out of the way (endshake) or if the hand is too short, or both then the cylinder hand can come out of engagement with the ratchet before the cylinder turns all the way to lock. That said, it would take a lot of endshake to cause a timing problem; my guess is they just checked everything and fixed anything that seemed out of spec. My guess is that the hand was out of spec. They probably tested it initially firing fairly rapidly and that didn't show the problem.
