Powerscaling is an almost completely subjective activity. You could easily compare it to literally analysis in this regard, except that literary analysis is less subjective. Most authors will know what themes they're going for in their work, what characters are doing, etc. (even if the quality of the work in question is not guaranteed in any regard). However, most authors probably won't give a second thought to how powerful their characters are and to what level their feats are meant to be placed at, beyond a very general outline. This is true of even combat-heavy works.
And then you have works with multiple authors, multiple interpretations/canons and straight up retcons, which make the whole shebang even worse.