The battery takes some time to, "freshen," up when you first start using it. As in, longer than three days. Not only have you not used the device enough when you were on 2.1, but you probably haven't used the device enough to get a good idea of your battery life while on 2.2 either. You have to use it for a little while before getting better battery life and a better reading. I did, anyway, and apparently so did others.
Also, YRMV.