Mosquitos don't fly very fast or very far, and they breed extremely fast, so if you have nearby puddles you get a "local high density" zone. The inflow is so large that it never settles down to the expected steady state with uniform density.

It's the same reason you have a high density of sports fans when you stand near the exit of a stadium after a game. The people (mosquitos) are streaming out of the stadium (standing water) so fast that there's a local high-density zone.

I literally live in a swamp, so I reasoned "how much could one tire hurt?" Oh boy was I wrong! Eliminating that one single mosquito breeding site near the house made an enormous difference on the local mosquito density.