My battery for my nookcolor went from 99% to 92% after not that long of use, is that normal?

This morning i used my Nook for about 20 minutes using Pandora and exploring barnes and nobles. Then about 6 hours later i used it again playing Sudoko for about 20 minutes. And when i saw my battery this morning it was 99%, and it's now down to 92%. and i didnt plug it in after i used it in this morning. I just dont want my battery to die so quickly. i am just a worried person! So is this normal? i also didnt turn my wifi off.