So, the plot thickens! The Bob Jackson photo was released, and it won the Pulitzer Prize! It showed Leavelle's hand disappearing into Oswald's pants.
Notice that you see up to the big knuckle of Leavelle's left hand. Where are his fingers? They are, presumably, buried in Oswald's pants. And they weren't claiming that Leavelle did that at the last second, rather, they were claiming that they were walking along that way, with Leavelle's hand gripping the "waistline" of Oswald's pants. And that's why they had to come up with this:
Think of it as B following A. Jackson's photo was A. People wondered about what his hand was doing, and Officialdom tried to explain it. And after explaining it, Officialdom went back and modified the footage so that it would support what they were saying about the Jackson photo. It was a very complex deception.