That is actually within acceptable limits. You can try reconditioning your battery though. Charge it up till green light on the magsafe, then run it off the battery until it goes flat without plugging back in. Then give it a full charge all in one hit. See if that makes any difference.
The battery and the laptop keep track of how much charge is left by keeping track of how much charge goes in and out. It cannot measure the charge present in the battery directly. Consequently it gets a bit confused and loses track sometimes. The process described above should allow the system to recalibrate itself.