What Field of View (FOV) actually is?
FOV is angle between opposite side of the image and your eyes.
There are horizontal and vertical FOV. For FPV Goggles manufactures usually use diagonal FOV which is even more confusing.
It is measured in degrees. The higher the FOV, the wider image you see.
Because of the fact that FOV is diagonal, the picture aspect ratio affects amount of visible area. A screen with 4:3 ratio looks larger then a screen with 16:9 with the same FOV value.
In fact, using the view mode different then native for the goggles, makes the effective FOV value lower. For example, if you use 4:3 image inside 16:9 screen of the googles, the visible image will have smaler FOV then goggle's default.