It's because of the optical mechanics. Mary Moorman was facing Elm Street squarely, and her picture was straight perpendicular, not diagonal.
Because of Mary's perpendicularity, her image would show Hargis ahead of Martin. That's if she took it then. But, I actually think that Mary took her picture before the Kennedys reached her. After all, why not capture their faces?
Here is Zapruder 292. Mary is set up. She said she was looking through the viewfinder all along, from the moment they rounded the corner at the top of the hill. So, why wouldn't she click the shutter?
Why the hell would she wait until they passed her? She wouldn't. She couldn't. She didn't.
This is Zapruder 300, and it is the most commonly claimed frame to correspond to when Mary snapped her picture.
What I like about this frame is that Mary's camera seems to be trained on the Kennedys. She is pointing her camera right at them. After that, they go on their merry way, and Mary remains frozen like that, never turning to her left to stay on them. For example, here is Z-309, and Mary is still frozen in the same position, as she ignores the Kennedys.
So, the Zapruder is completely fucked up when it comes to Mary Moorman. But, let's go back to 300.
That is the last frame at which Mary could have taken her picture. She may have taken it before that, but she certainly didn't take it after that. But, where are Hargis and Margin?
So, Mary's camera would have seen Hargis before Martin and captured his (Hargis') wheel in front. They knew she was used to seeing that, so they adapted the frame from Babushka accordingly, installing the white thumbprint to take out Martin.
Now, keep in mind that the Punk tried to get away with a lie, claiming that thumbprint involved Featherstone, but it didn't. We have Mary telling us herself that the thumbprint materialized when the FBI borrowed her photo for the second time. It was weeks after the assassination! And the plain truth is that there not even a theoretical basis for how such a thing could happen accidentally. They did it on purpose.
The big question is: what did Mary's own photo reveal that they considered to be so damaging to the official story?
I can only speculate about that. I don't think it's some arcane shooter up on the hill because that they could have easily taken out. It had to be something in the limo, either with the Kennedys or the Connallys or both.