It doesn't seem to me like there is overwhelming INTJ representation on this site - I wish there were more, since they are so uncommon IRL. I don't know how long you've been lurking, but perhaps there has been a recent surge in INTJ posting, and drop in ISFP posting. I feel like there is decent ISFP representation.
As for mistakes in self-typing, that's quite common, though in general I think most "real" INTJs are quick to identify themselves as INTJ, being as information-absorbing and decisive as they are. Perhaps some INTJs on here are really something else, but that's true for all types. I dunno, in general you can usually tell with people because types have posting quirks. INTJs are generally pretty informative/instructive in a benevolent "teaching" sort of way, succinct and sometimes vague, and often serious interspersed with goofy. Nico, Oaky, Uumlau, Z, Kalach all basically follow that pattern, with some individual variation. INTPs are generally more wavering and long-winded, with more questioning (and more observations, with more mature INTPs). They seem to pick things apart more and INTJs seem to synthesize more. Often when someone is identifying as a different type than you think they might be, there are certain "tells" that set them apart... and generally eventually they will question their own self-typing. It is true that certain types are touted as "better"... but that has a lot to do with personal taste, too. I'd personally love to be INTJ or ISTP but they're so radically far from my personality that I couldn't delude myself into thinking I was one if I tried.
Regardless, type is "best-fit" and can only be self-determined anyway. I don't feel like there's any objective truth to be had, just guesstimations. If someone doesn't fall into the category you think they should... well, that's up to you whether you accept their self-typing and change your conception of that type to meet that person or stick with your conception of type as it is and classify them as something else.