There seems to be some confusing regarding how to under PFP and whether it makes sense as a concept. This confusion is, well, confusing because there is an intuitive and coherent understanding of the concept:
PFP is about comparing fighters across two dimensions: First, the level of the competition in the top of a weight class and how much better they are than that competition. Second, how the level of competition in weight classes approximate to them, and how well they could fare against that competition.
For example, Sean Strickland should not rank very highly on the PFP list. Because while the top level of competition in his weightclass is decent, he is not that much better than the rest of the division. And the top of he would not fare well against the top 5 in the weight class above him, while he simultaneously would not have a walk in the park with the top 5 in the division below him.
With this understanding in mind we can understand why Volkanovski should be higher on the PFP list than Islam, even though he lost to Islam. Because while the top of featherweight and lightweight are equally strong, and the distance between Volk and Islam and the rest of their compared division is equally big, Volkanvoski perform better in the "level of competition in weight classes approximate to them, and how well they could fare against that competition". Volkanovski would not walk through the champion in the weight classes below him, while we have already seen this to not be the case for Islam. Similarly, Volkanovski is, likely, more competitive against the top 5 in the weight class above him (we have already seen him be competitive against the champion), then Islam is.
PFP is about comparing fighters across two dimensions: First, the level of the competition in the top of a weight class and how much better they are than that competition. Second, how the level of competition in weight classes approximate to them, and how well they could fare against that competition.
For example, Sean Strickland should not rank very highly on the PFP list. Because while the top level of competition in his weightclass is decent, he is not that much better than the rest of the division. And the top of he would not fare well against the top 5 in the weight class above him, while he simultaneously would not have a walk in the park with the top 5 in the division below him.
With this understanding in mind we can understand why Volkanovski should be higher on the PFP list than Islam, even though he lost to Islam. Because while the top of featherweight and lightweight are equally strong, and the distance between Volk and Islam and the rest of their compared division is equally big, Volkanvoski perform better in the "level of competition in weight classes approximate to them, and how well they could fare against that competition". Volkanovski would not walk through the champion in the weight classes below him, while we have already seen this to not be the case for Islam. Similarly, Volkanovski is, likely, more competitive against the top 5 in the weight class above him (we have already seen him be competitive against the champion), then Islam is.