Board Thread:Questions and Answers/@comment-26005711-20151226104253/@comment-26871105-20161004055450

It's a running gag in HxH that practitioners Shingen-ryu for some reason, seem to appear (or behave in such a way that might give the illusion of being) much weaker than they would otherwise really be.

Hence it's entirely possible that Hisoka never quite know how strong Netero was, or may have even assigned him with a clearly incorrect and lesser score. But we don't know.