Map Enthusiasts

I'm confused about what this map is actually saying. This is basically just the max population city on any given long. But that's the other thing, what's the east west cutoff? There's multiple settlements from Russia, so it's not per country. Lots of those settlements are much closer together to each other than say, new York and Seattle or LA or whatever it would end up being. What's the margin of degrees on east west?

Best I can tell, the data set must have been "we picked some random recognizable and unheard of settlements and called it good."

Also the geographic north pole isn't marked on the map, which adds to the confusion. The lat/long don't even go to the north pole, or anywhere close to it! That's arguably the most important reference point of the axes...

Maybe someone who understands the map better can explain it better, but from the information I can see it makes no sense and doesn't seem to tell me anything.