Snuffy was referring to the other axis of the scale, for balance beams. Your "standard" axis is the Z axis, for zeroing the scale. However, if it is also out of level in the X axis, it causes drag on parts of the scale that should not be touching.
Thanks Frank, there ARE two axis to being level. Why not have both level, then you know for sure there's no off center force on the beam, or plenum of a digital.
Take a level to whatever base your scale sits on, first along the length of the scale, then at 90 degrees to that. Both should be in the center of the glass.