There could be a dead cell in it, or it could simply need to be calibrated... I think the process for recalibrating it goes like this;
Charge it as far as you can, then run it comletely out of juice, dont even let the thing go to sleep. just run it on high contrast no screen saver until the battery is completely dead, even when it shuts itself off, keep turning it back on until it simply wont let you. THen charge the battery to full when the computer is off and try turning it back on again. If this doesnt help recalibrate it, then the battery might be starting to go. Never seen anyone make a laptop battery last the way its supposed to. Also really depends on your habits, ie: if you leave it on power all the time, let it charge fully before use/unplugging it all the time and etc.