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.