Measuring a diameter on a Torus at a fixed location

[[Ri...]]

Calypso 6.2.1x
Win7
See attached image.

I'm struggling to get factual results from scanning a Torus.
The model is stable and correct.
At first setup a "Circle on Torus" will use the ID of an OD feature, beware.
After correcting that I noticed the Stylus center doesn't set on the center of circle to be scanned, its offset.
I found out how to correct that (I think).
The one thing that stands out in any configuration, the scanned diameter is never the same as Nominal, I'm on an off line seat and it should be 100% of my nominal value. I'm guessing there is some sine error somewhere.

Are there some special rules to measuring features of an Torus?

Circle on Torus Issue.jpg

[[An...]]

I'm really not sure.
Perhaps Zeiss might confirm.
See attached.

Circle_on_Torus_1.pdf