The different values are different measurements. The screen is not a square, so you will have different vertical and horizontal measurements. The 90 degree value is the horizontal measurement if the screen was 4:3 wide.
90 FOV in CSGO means 90 horizontal degrees in 4:3 aspect ratio. If you are using a wider AR, the extra width is added on resulting in 106.26 horizontal degrees for 16:9.
And yes, 73.74 is what you need to match this in games that use vertical FOV.
Well since Unturned's FOV is measured in Vertical degrees then you want to match it to your CSGO VFOV 73.74 so around 73-74 FOV in Unturned. Unless you can change it through the config to make it exactly 73.74 like it is in CSGO.