Two identical birds going in different directions will obviously require the same amount of forward allowance if they are going to be hit.
However, your question isn’t as illogical as it might first seem.
The lead picture can appear slightly different particularly among shooters who rely on speed of swing to break targets as with pull-away or follow through techniques.
A right-handed shooter taking a right-to-left crosser enjoys the benefit of the stock coming into his face as the swing of the gun progresses and he’s better able to judge speed of swing and forward allowance as a consequence.
The opposite applies to a left-hander.
In the case of a left-to-right crosser, however, the trajectory has the ability to take the stock away from the right-hander’s face so a natural reaction is to speed up the swing to avoid the possibility of the gun leaving the cheek.
As a result of the slightly faster gun speed, the apparent amount of lead appears smaller.