I would guess that it just needs recalibrated, yes.
Although it may also just be that the stock can't be that accurate. I have another app replacing it so I can't check, but say the entire thing is four pixels wide. For 100% of course it'd have all four pixels. But it may wait until it's below 75% to only show three pixels. Until then all you know it it's somewhere between 75 and 100%. Same with three bars - it could be anywhere from 74 to 50% battery life. It's just that the app's size limits its accuracy.