Because there was TWO angles.Ever heard of depth of field? How did the review dude know that from a tv replay?
It is exactly the same thing with our human vision. One eye can't do it, two eyes working together can.
Look at the diagram again. Try to place the footy somewhere, other than on the post, such that lines drawn from each of the cameras BOTH go through the footy, and BOTH go through the post. Try it. Screenshot your solution and paste it here.
By knowing the ball was actually over the post when the TWO images show it to be from there perspective, we know it was ACTUALLY over the post at that time, and as such, we know the depth, we know it was over the goal line, etc, etc.



