Compass question

[Bekkr]

I think maybe I put this in the wrong forum before... I'll try again here:

I've downloaded quite a few compass pieces over the past months, and one thing they all seem to have in common is that they're off by varying amounts. What I mean is, with the compass reading due north, when hovering the mouse over them, I've yet to find one that will read zero degrees. Some are out by only a few (3-5) degrees, but others are out by as much as 20 (one in particular that I like the look of is out by nearly 100). Does anyone know why this is? And is it fixable, or just a quirk of the UI?

Thanks in advance,
-B.