In 2003, the viscosity scale was revised. They took the old 75w-90 range and split it in half; the thinner half retained the 75w-90 label, and the thicker half was re-labeled as 75w-110.
If you take a look at the gear lube chart link that Pablo posted, you can see the old rating scale vs. the new (click the big blue boxes at the bottom of the chart). It's a great illustration.
So basically, if you see a 75w-110 today, it would have been called a 75w-90 back then. Conversely, any oil that was on the thick side of the 75w-90 range back then would be called a 75w-110 today.
Amsoil MCV is 20.1 cSt @ 100 degrees C, which makes it equivalent to a 75w-110. On the pre-2003 scale, though, it would have been called a 75w-90.
Does that make sense?