OK.
So I found something (it is an a private thingy, so I cannot give you a link).
ISFJs can mistype as INFP/ENFP because, if an ISFJ is a quirky person or is different from others, they will be hyper aware of it. They will notice that they are different/weird compared to others, and since the usual/stereotypical description of type is that those with Fi are different or weird, then they mistype as INFP/ENFP. If a true INFP/ENFP was weird ...they actually wouldn't notice as much as an ISFJ/ESFJ would. In fact, ESFJs and potentially ISFJs can manipulate their "weirdness" because they are good at reading social cues (or at least better at it then an INFP ...despite how "bad" the ISFJ/ESFJ thinks they are at being normal).
Also, ISFJs are typed as 6s often. Being type 6 is already such a complex type ...and only ENFP/INFPs get called out as being complex. INFJs also think they are complex ...but ISFJs have Si-Ne, so they are more likely to mistype as ENFP/INFP.
Also, in today's society ...almost no one is traditional anymore, so we cannot use this "weirdness"/"quirky"/"unique" thing as a basis for typing someone.