So I took a look at CSGO and fov_cs_debug now. And sure enough, when I change FOV, the base sensitivity changes. Just changing to 91 makes a huge difference in 360 distance. But changing from 91 to 100 scales like expected. So I was a little bit confused, as I think people would have noticed this... Then I tested on another machine with a brand new CSGO installation, and there it scales like expected and according to the calculator; changing to 91 makes just a very small difference.
The CSGO installation that scales wrong have been used for a lot of r_input testing, analysis with different variables and so on. So I have somewhere set a variable or switch that makes FOV scale differently. I'll look through the config file and see if I can find the cause, but unless you have the same "problem", the calculator is correct.