Another reason might be that (I'm not sure if anyone has noticed this or not), but the Music Star is a tiny bit thinner than the Tama-go; It also "rests" (like when you put it down on a table) on its back, with not much thickness. (Everyone following?) It's easier to get to the circuit board because its so thin.
In Binary's log, he mentioned that there's some extra space between the circuit board and the actual Tama, therefore making it harder for liquids (such as orange juice) to seep through the cracks and get to it. If you took your Tama-go out of the cup really fast, it's possible that the juice didn't actually get to the circuit board.