When you set your FOV, this fixes your vertical FOV. But the number you set is the horizontal FOV that you would have with a 4:3 screen. On a widescreen monitor, your actual horizontal FOV is larger than your FOV setting.
Here is the math for it:
Let R be your aspect ratio and θ your FOV setting.
θ' = 2*atan(R * (3/4) * tan(θ/2))
θ' is your actual horizontal FOV.
θ' for common aspect ratios and 90 FOV setting:
5:4 - 86.3
4:3 - 90
16:10 - 100.4
16:9 - 106.3