... Apparently with music, and in essence not with pretty much anything else (nobody quite knows why), when you run through a piece you know well, just in your mind, your brain triggers all kinds of flashes and systems that one would usually associate with motor activity, even though your fingers and such don't actually move.
In other words, memorizing music in terms of performance – that is, memorizing a piece to play it rather than just by listening to it – appears to get literally embedded in motor areas of the brain. This does not appear to happen to a significant degree, nor with this level of complexity, with other kinds of memorized systems.
And it's not just motor stuff either: music triggers weird systems and regions of the brain in ways that are wildly atypical and pretty much unlike anything else. Nobody knows why music should be this way, but it has lots of manifestations.