It says that CS:GO is on Hdeg 4:3, so what does that exacly mean? Does that mean the "Actual HFOV" is 106.26 when you play on 4:3 or just on native 16:9?

It means that the FOV value is what the FOV is for a 4:3 aspect ratio. When your aspect ratio is wider, the additional width is added on, resulting in an actual FOV of 106.25 for 16:9.