My theory:

There's no such thing as "muscle memory" (in terms of the muscles themselves "remembering" a motion), but your brain can get used to a movement. So think of it as long term versus short term memory. Once it's in your long term memory, you're unlikely to forget it. While it's in your short term memory, however, there is a lot of data and information constantly being lost.

So you train and train and train a movement over the course of a day, and you finally get it, you're all happy and you can do it a few times. But then you wait for a few days and you try it again. You KNOW you can do it, but your body doesn't entirely remember how to do it. I've noticed this when I was learning my back tucks, and now that both of my roommates are working on theirs. You make huge gains, and then you go back a few days later, and you've made negative progress. But after a few of these, you get comfortable with the movement, you remember it, and you become much more confident in being able to REPEATEDLY perform it, as opposed to just once or twice.

Any input on my theory?