I get that some people will have better technique and we don't cast exactly the same each time,and this is just a Fag packet theory that could probably never be proved,
but in a perfect situation with everything identical, I think with a shorter rod the lead would go to point B closer to the water more directly, meaning it would arrive at point b and continue to travel towards the bottom and push further under the tree as the angle it enters the water would mean it would continue further than a longer rod which will have travelled in a bigger arc meaning it will enter the water at a bigger angle and drop shorter than the short rod
I don't know if I can explain it much better than that but. That's my theory