I've had the same behaviour. I tend to get a good battery performance with one charge and considerably poorer performance with the next charge so there is some sort of fluctuation. Could be due to the fact that at work I charge with charge adapter while at home with the dedicated charger.

But I've also noticed that leaving browser windows running in background sometimes drains the battery pretty fast. Fortunately it seems that the fix to wlan hanging using static ip's works for me.