Curious as to what happened that they couldn't get the shark near his back to make that a little more accurate as to the body damage.
Well it was 1975, still well at least two years before most even had the first VHS, so they weren't thinking people were going to be able to repeatedly rewatch, rewind and scrutinize every shot, let alone do screen captures and grab frames out of context.
They were likely thinking most would be screaming, and covering their eyes anyway, and were just going for the most dramatic one-viewing visual impact.
Even then, as far as I can tell the scene Is actually incredibly well staged, considering what they were working with:
Looks like he takes one first chomp, pulling him into the water, that cuts off the leg, then goes for the bigger morsel, the full torso, leaving the now severed leg to drift down to the bottom. Or that second chomp at the torso, leaves the other leg to drift down.
Don't know if it even needs spoilers, but just in case.....