Battery Reporting Working?!?!

Ok, so i was running android and i just woke up left it running it last night. I unplug it and the battery is yellow and at about 20% it looks like. I open up the battery indicator, and it tells me it is at 18%, then 15%, then 5% and which point it dies. So now i have to wait the 10-15 mins to get it to turn back on, but im not complaining about that. What I want to know is how was it able to read different %'s when it is about to die, but not when its full?

(im not complaining about anything, i know its still in alpha/beta stage, just curious as to how it can "work" when its dying.