I've recently taken up the art of learning American Sign Language. It's something I've always been interested in, and I've tried to learn twice before, but lacked easy access to people with whom I could practice. Now I have at least two friends who sign, and I'm paying for school anyway (getting my graduate degree), so I figured, why not?
I really like the instructor. She's very personable and extremely expressive, which makes understanding her signs much, much easier. What I've come to realize, though, is that everyone signs things just a little bit differently. Even when using the same gesture, two people can employ subtle differences in precisely how they bend/twist/shape their hands. I think of them as visual accents.
What's interesting - and the tie-in to dog training - is realizing just how dependent I am on the instructor's specific "accent." I find that I sometimes struggle to understand other signers when they layer their own "accent" onto a sign. ToMAYto, toMAHto? Yes - but not when you're first learning.
Think of your dog during an obedience class. You think he "knows" sit (or whatever behavior you're working on) because you do it all the time. But when you practice, you generally have the habit of standing directly in front of your dog. Your body's "accent" is to be in front of the dog, and he is keenly aware of this. Later, when he's standing in proper heel position at your left side, and you ask him to sit, you've changed your body's "accent" and he's likely to struggle with the "sit" behavior.
Of course, the more you practice having your dog sit - in lots of different positions relative to you - the quicker he'll generalize the behavior (that "sit" means "butt-on-floor" no matter where he is in relation to you). Eventually, he'll become fluent at sitting - meaning it will happen anywhere, any time and it's never confused with a different cue.
For me, the more people with whom I practice signing, the sooner I'll begin to fully generalize the hand shapes and won't be as thrown by subtle visual differences. I don't know that I'll ever become truly fluent, but much like when training a dog, you don't know what you're truly capable of until you try!