I have the same problem. I'm assuming computer is still calculating how long you have, it generally updates with time. I set power saver the other day and I had gone from 6 hours to 5, and then up to 8.
I wouldn't worry, if you lower the brightness and volume then you are going to last longer, simples.
It means that your power saver button is not working.So try to install the power saver driver again.