I think it's recoil. It might possibly be the time of flight. For comparison, a mistake of 5 mph in speed estimation or a mistake of 5 yards in distance estimation will put you off one whole foot. But this is on a pure crosser. As the bird starts to angle, the difference shrinks.
Difference becomes zero on a straight away.
I don't think that this is the real answer, but it's a consideration. Recoil is where the little 24 gram metric load really makes sense. There is nothing magical about it ballistically. It's clearly inferior ballistically. All other things being equal, has to be, got to be.