There was a post somewhere about this. Basically, I think you're seeing the management system kicking in.
It's not good for Li-ion batteries to be 0% or 100% charged for any length of time, so the post I recall reading was hypothesizing that it will take the battery to 100%, then let it run down a little wait, and bring it slowly back up. It also mentioned that the "reporting" system was doing some weird things, too.
I've noticed this somewhat on my S, too. Some mornings it's at 100% and others it's only at 97%. It also seems to drop from 100% - 90% really fast, then it slows up. In any case, I always make it to the end of the day with anywhere from 40% to 25%, depending how the Word Feud battles are going. (And in my case, they are never going too well)