INFJs are supposed to be rare. Maybe it's possible that when research was done to find the percentage number of each type, INFJs were only rare because they were the hardest to actually type.
SO catching the INFJ in their original state was really hard. INFJs can easily mislead people into thinking they are any other personality type.
I'm just saying INFJs could be more common than we think they actually are. I'm not sure exactly how the research was carried out so if they had their own 'full-proof' method to determining the INFJ then I could be wrong.