Originally Posted by GeoffR
Others have suggested that the battery meter needs this in order to calibrate itself.
Indeed, the battery needs a few cycles (from completely empty to completely full) for the battery meter to calibrate to it properly. Also, the battery life will get slightly better after about 4 cycles. So don't worry too much about battery life and especially not about the meter until you've had at least 4 cycles.
Also, my battery was 'stuck' on 81% for a day and a half or so, even during reading, until yesterday it dropped to 80% while I was reading. So it does update while you're in a book; your meter just probably needs calibrating.