Here's a pic to show what I mean. Ignore the orientation arrorw, it's set to my local declination.

If I draw a line from center of compass to the 0, +90 and -90 of the declination scale, it doesn't intersect with the corresponding S, E, W marking on outer ring, but off by 1 or 2 degrees. So either the vial or the outer ring is misaligned, and if I were to set the declination according to the scale, 0 wouldn't be quite 0, and my reading would be off by a degree or two, or am I over thinking it?