That's more of a reporting issue than anything else. You can clear the battery stats by rebooting to recovery, but fundamentally battery reporting is a black art. Based on the voltage they're guessing how much charge is left in the battery but the voltage will vary depending on draw and the rate of discharge isn't exactly linear either. In short, battery reporting is more of a rough estimate than anything you should really be too concerned about.
There is also a way to report 1% increments, but there's a good lesson regarding accuracy vs. precision in there