It comes down to relative value of closing the deal. If both A and B want to increase their X, whatever it may be, an increase of 3 to A's 4 is not as much as 3

It comes down to relative value of closing the deal. If both A and B want to increase their X, whatever it may be, an increase of 3 to A's 4 is not as much as 3 to B's 2 if we're talking about market share as an example. So already it's of more relative value for B to close the deal with +3's. Other factors apply. So it really comes down to how much each wants it, which is the status quo for negotiation. I don't think any business goes in saying "What's the fairest way we can make this deal?" As an edge case, let's say A has 1M, and B has 1k and there's 500k new units. Should B get 250k same as A? Would A go for that? Would B be happy with 100k?