Gravity variation with position on Earth
Another thing to consider is that gravity is not the same at every point on earth. The biggest variation is with lattitude. Altitude also has a small influence.
From Wikipedia:
"... the equatorial bulge and the effects of centrifugal force mean that sea-level gravitational acceleration increases from about 9.780 m·s−2 at the Equator to about 9.832 m·s−2 at the poles, so an object will weigh about 0.5% more at the poles than at the Equator."
This variation is not as much as the difference between what was noted in the opening of this thread and the expected ~1.0, but it shows that there are other reasons besides sensor tolerance and sensor alignment that would lead to a measurement other than cosine(pitch attitude).