Where are the data coming from?
You cannot draw the conclusions that this article does without adequate data. Long tail niche products are sold primarily by long tail niche websites, not Amazon. There is absolutely no way to collect enough data on these websites (there are nearly as many of them as there are long tail keywords) to state anything conclusive at all about their revenues.
Oh well, academics are never bothered by reality. Remember the Efficient Market Theory? It states that you can't make money speculating in the stock market. It is favoured by academics, but not widely accepted by millionaire stock market speculators.
The reason why the data to not match the expected curve is simple: the data set is incomplete.