Because we're not gymnastically trained, Greg? lol
I don't know why you're so hostile about it, I can only count a 900 spin on it, I even got a spoon and reproduced your movements in 3D, watching your right hand side rather than your head and chest, and I worked it out to be 900.
The problem with the video is the angle is straight on, ideally it needs to be filmed from the side to count rotations so we can focus on a fixated point that only rotates in one direction (e.g. your hip).
The front view makes it hard to count as not only is your head/chest rotating around your vertical axis (from the rotation), but also round the horizontal axis of the flip. The side view would only rotate in one direction as the direction of the flip would be perpendicular (rather than parallel) to the viewpoint of the camera, so it wouldn't affect the view.
The two directions of rotation trick the brain as it looks like more than it really is, with 1 direction of rotation people would be able to count/see the rotations better.
I've attached a diagram if it helps explain all that better.
Incase that all confuses you, here it is simply put - The front view of the backflip, you see your face, then back, then face, which adds a 360 spin to it; From side view this wouldn't happen as we'd be focused on your side