In csgo you can change your viewmodel fov which changes how much you can see your weapon, arms, and hands. But after some testing I found out that this does not change your actual fov at all. So I'm confused that when I put 91 (horizontal/ 60 vertical) my sensitivity is 1.5 (converting from apex legends at 103 horizontal fov and 1.5 sens) my csgo sensitivity is 1.5, but if I use 100 horizantal fov (which is max bc max view model fov in cs is 68 vertical) my cs sens accoring to your calculator is 1.35. So why does the sens change if the fov is not changing?

Viewmodel FOV is as you say only the render FOV for the weapon etc. If you check the output for CSGO you'll see that the config FOV is fov_cs_debug, which does change your FOV. But it only works with sv_cheats enabled or playing on a server with cheats. If not, then 90 is the only correct value as the game will be locked to this.