Low Battery warning at 90%

I have a G4 ibook, that didn't get used for a long time, (over a year) and I am using again after my newer laptop was stolen. The battery may be near the end of it's life, but I am confused by getting a "low battery warning" dialog box, when the battery indicator says 90%.

If it was just dropping quickly I'd assume the battery was toast, but I don't get the conflict between the two pieces of information.

Is there any point in resetting the PMU? Should I just consider this a not-so-portable until I replace it?