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Hi! It was me who wrote that caption, however I did not say originally. ""My"" sentence is: ""The IQs of a large enough population are calculated so that they conform to a normal distribution."" I totally agree with you that this is not how it was done originally, but as far as I know, this is how it is done today. Unfortunately, I don't have a specific source for it, but it is basically what I have understood from reading lots of intelligence-test-manuals, books on assessment and intelligence, etc. 
There is no research that says that ""amount of intelligence"" is distributed across the population in the same way as a normal distribution, but this is the way IQ-tests are made.
For example, if a small country suddenly gets a high influx of highly intelligent immigrants, suddenly more than 50% of the population would have an IQ of 100 or higher. But instead of changing the normal distribution, the next time when they standardize the IQ-test, they take a representative sample of 1000 or 10,000 inhabitants, and the ones that have an average intelligence get IQ 100, the 2% most intelligent people get IQ 130 or more; the 2% least intelligent people get IQ 70 or less.
This is how I have understood the statistics behind IQ. However, I am not a statistics specialist at all, so please correct me if I am wrong!  talk "