Iām 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 ācenterā 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 doesnāt mean āspawn one zombieā, but more like ā1/10 chance of spawning a few zombiesā. 0 however, does mean āno zombiesā. Thatās the plan at least, itās still in progress 
So if you have a town like this (crappy town yes, but Iām only using easy to type stuff):
The ā!ā is the ātown centerā.
....+..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
Itās more blobby and conforms to the actual shape of the town instead of an idealized circle.
Please donāt 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 āsuburbsā jutting out from the town that you could count on to be totally empty, the balance point weāre looking for (and still working on) is have groups of houses jutting out like that be quite sparsely populated, but not known āsafeā zones.
Just a reminder, this is not the actual output of the system, itās 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.