I’m not totally sure about numbers, but we recently changed how spawn densities are mapped so it conforms to where the buildings are instead of being centered on the “center” of the town. Spawn density is a number that represents both how likely a group of zombies is to spawn at a location, and if it does how big it is, so a 1 doesn’t mean “spawn one zombie”, but more like “1/10 chance of spawning a few zombies”. 0 however, does mean “no zombies”. That’s the plan at least, it’s still in progress 
So if you have a town like this (crappy town yes, but I’m only using easy to type stuff):
The ‘!’ is the “town center”.
....+..FFFF
....|......
....|..V...
...>|<>|<..
...>|.>|...
....!<.|<..
...>|.>|...
....|__|...
....|......
The old density would be something like:
00000000000
00011100000
00112110000
01123211000
01234321000
01123211000
00112110000
00011100000
00000000000
You can see the circle, and that some of the houses have a density of 0, so no zombies there.
And the new density would be more like:
00000000000
00000011100
00112233110
00223344210
00224444220
00223333110
00112222110
00111111000
00000000000
It’s more blobby and conforms to the actual shape of the town instead of an idealized circle.
Please don’t focus on the fact that the numbers are higher, and more spread out, the point is that the old method drew circles around a basically arbitrary point, and the new one tallies up what structures are nearby and uses that to determine gow big groups of zombies will be. Previously there would be “suburbs” jutting out from the town that you could count on to be totally empty, the balance point we’re looking for (and still working on) is have groups of houses jutting out like that be quite sparsely populated, but not known “safe” zones.
Just a reminder, this is not the actual output of the system, it’s a simplified version in that a house contributes 1 to its own square and to each adjacent square (think minesweeper mines). The real system has a variable number for different structures (houses are low, hospitals high, stores in the middle) and is spread out over more tiles, so the single suburb justting out would get even more diluted.