To me, what I am about to explain seems obvious, yet it is never been given lip service by the press or boxing pundits.
It relates to what happens to a boxer's p4p status as he moves up in weight.
Let's take Pacquaio. I have seen it written that by moving up and defeating de la Hoya at welterweight, he has solidified his hold on the p4p #1 spot. I would say he has solidified nothing. Now that he is at welterweight, I don't think anyone is going to say that he is the #1 welterweight, that he can beat guys like Cotto, Margarito, Mosely. But it doesn't seem like anyone is going to take away his p4p#1 either.
You could say the same about a lot of guys. Hatton moves up to welterweight and wins against a lesser champ. Suppose he stays for a while. His p4p doesn't go down, even though is is now in a much tougher division, in among a number of fighters that would be favored over him.
Probably one could come with a similar argument for mayweather.
The whole point is, the whole basis for a particular p4p ranking changes when a fighter changes division.
No big deal I guess. The whole concept of p4p ranking is pretty questionable anyway.