iPod Battery Level Is "approximate"
From the iPod support pages.
Note: The iPod battery indicator shows approximately how much charge is left in the battery. In some iPods, you may find that the battery indicator shows some charge left, but your iPod stops playing because the battery is, in fact, empty. In other cases, the battery indicator may show less than a full charge even though you've fully charged it.
I always suspected this.
And find it really irritating.