I agree that PBF being the current great and having ended his career at WW now he is being compared to past great WWs instead of past SFW or LWs. In all honesty a better comparison would be PBF vs Mosley at LW or PBF vs Oscar at LW.

Fighters are often wrongly judged by the fights they had at the end of their careers at weights they don't belong in the first place. As good as Oscar was at welterweigh, he was much better at the lighter weights. Other fighters like Trinidad are also judged like this.