Yeah it's a fair gripe really, and I've no idea as to whether a Heavyweight has ever been ranked at no.1 for any length of time.

However, from what I've read before, the whole concept of the ring having a p4p ranking initially was meant to indicate who the best fighter in the world was ASIDE from the Heavyweight champion. As the division has fallen into shambles in recent years it's kind of a moot point as your hard pressed to find a HW who deserves to be ranked in the top 10 anyways imo... But perhaps they were meant to be exempt to begin with, as it was a given that the best heavyweights around were the best fighters in the world at one point.

Furthermore, if you were to somehow compile a p4p list of athletes from ALL sports, I think it would consist of almost entirely of heavyweights, due to biomechanics alone, but that's up for debate to some extent, and not worth debating really. Fact is HW boxing hasn't had anyone you could really chalk up with a Lebron James type of athlete etc. since the 90's at best(really ever imo, but whatever), and even then it's extremely unlikely. But it's a whole can of worms not worth opening, the debate just doesn't work. P4P lists are never going to be concrete due to the vast differences in physical attributes between men of a given size.