Or maybe the statistics are wrong and INFJs aren't actually rare. Idk how the sites that claim 'XXXX types make up X percent of the population' could really even know that, anyway.
A lot of threads are using just US data, but what basically happens is that there is an official MBTI running as business, and they, probably encouraged by Big Five, are doing psychometrics. They basically register some people who did business with them and select random samples and combine this (or maybe it was just some random samples) to try to tackle the country's MBTI distribution.
I had done a thread compiling all their work in a single image - two images actually. I had also made a Global projection of % types in whole world population by considering country population as weights - for example, highly weighting China - and by using some few country to extra-represent the continent, and by that I had reached that about 2% of people MBTI type is INFJ in the whole world, making them the rarest type.
I had also found later, on a more limited way, that from the intuitives and ISFP/ISTP, ENFJ and INFJ are the champions in terms of borderline. But..
As we know, in typology communities the numbers of INFJs and INFPs are more or less the same, and we know that there is a tendency on the forums to have more NPs than NJs, but that mirrors real life. And about that, in real life there are about 2 INFPs per INFJ. And I also know that the most common borderline for INFJ is J/P. With that all information, you are able to easily induct (from inductive reasoning) that is very likely that a significant number of INFJs are actually INFPs with borderline P/J instead. Probably because INFJ seems more attractive on profile descriptions and on Big Five than INFP.
16Personalities and some few uses the test takers as data, but these are biased because INFPs and their cousins plus INTJ have a tendency to take these tests way more than Extraverted Sensors. Although INFJ is rare on real life, they are quite common on forums like this.
So, yeah, that didnt came out of nowhere.