What is your alternative assumption? You say that the rest of the thread makes it clear that "they are not" analogous. But I'm not sure that the basic argument about difference in the size of the fanbase makes any difference to the buying behaviors of those fanbases.
For example, you said that you were not buying from amazon because you want to support your local gamestore and you are waiting. Are you suggesting that there is a widespread presence of Paizo shoppers (you did not specify what product you ordered) who are deeply loyal to their local game store and a notable lack of the same amongst 5E fans? And are you suggesting that not only is that true, but the scale of this is such that it can drive a visible divergence in Amazon data? I find the first point extremely unlikely. The second point can't happen without the first, and even if we just assume it for the sake of argument, the second point still seems beyond reason.
Do you have another theory that simply has not been explained yet?