I noticed the FOV in Unity is only correct when the screen resolution is square, in other words when the width of the screen is the same as the height.
When you add more width to the screen, the camera shows a greater FOV sideways. (The FOV remains the same up and down.)
To see this is true, set the resolution to be square, set the FOV to be 90, and go into a corner and look out and compare the positions of both walls: They will match the sides of the screen.
So even 4:3 screens show more FOV. 16:9 shows way more. I am not sure of the math to determine how much.
I wonder what aspect ratio shows a FOV of 90 (sideways) when it is set to the default 60? Does anyone know the math?