Sounds a little odd. But then they are a little odd based on what you're doing. My Glo when FULLY charged stays on 100% for a while if just reading, then drops off gradually. Accessing TOC and bouncing around will bring it down much quicker, font size changes and the like as well. It's normal for WiFi to drag it down and easily knock off 5 points of the battery percentage with one sync, in my experience.
Perhaps you forgot to turn the WiFi off after downloading and starting reading again.
Frankly, any processing other than simply turning pages can contribute to fairly rapid discharge.
I recently did a discharge test with my Glo in sleep mode and got 46.5 hours of reading time over 24 days and 4 hours, so that might give you something to shot for. Someone else reported around 44 hours of up time over about a two week period, obviously reading a bit more a day than I averaged.