Any study on type frequency has to take into account who the sample population is, region, class, etc. If we tested the employees of a software company, for example, we'd most certainly find a larger percentage of NTs than would be found in the general population.
Also, we can complain about all of the "fake" INFJs on internet forums, but it is likely there is going to be a much larger representation of them found in online communities, disproportionate to "real world" frequency of that type, as NFs tend to be driven to be identity seekers or to learn about other types. In general, this seems to hold true for INTs and INFs on the internet. A gross overgeneralization not meant to offend or stereotype, but if an NT or NF takes a personality test, I'm willing to bet they are far more likely to want to learn more, to keep digging--especially INFPs and INFJs. A S-J or S-P might say, "cool, I'm this type", accept the results, and go on with their life, hence why they are a greater part of the general population but tend to be unicorns and diamonds in internet typology communities.