It looks to me like the (spoilers for coordinates)
strange frequency distributions seen in non-longitude coordinates is a lot like what you get from a normal distribution minus another normal distribution, with lower standard deviation, scaled down so that its max is equal to the first’s max. I feel like I’ve seen this … vibe, I guess, from curves, when I have said “this looks like a mixture of a normal distribution and something else” and then tried to subtract out the normal part.
I began by looking at what the coordinates must mean and what the selection bias implied about geography and (obviously) got hard stuck.