The rules clearly state in all three manuals:
"The Mission plans show where Zombies appear at the end of each turn: these are the Spawn Zones. Chose a Spawn Zone and draw a card. Place the amount of Zombies which is shown on the color that corresponds to the Danger Level of the most experienced Survivor still in the game (Blue, Yellow, Orange, or Red). Repeat this operation for each Spawn Zone."
The last line is our error.
What we have been doing is drawing a single card, choosing the appropriate danger level and assigning that amount of spawned zombies to ALL the spawn points. In playing thus we, through our ignorance and arrogance, fail to turn over enough of the Zombie action cards during a game, thereby decreasing the chance for an Abomination to appear. On the other hand we have increased our chances for the Zombies to get an "extra move/attack" as when there are not enough minis to complete a spawn properly all Zombies of the spawn type get and extra action.
I have mocked up the following example to show how by drawing a card for each spawn point changes how we will have to adjust our play style and thinking.
Survivor Joshua has bumped his experience level up to the Orange bar by clearing the board of a whole slew of walkers. Thus on the Zombie turn all spawns will reflect that experience level i.e. Orange.
There are three Spawn points on the board, so a card is drawn during Zombie phase and as the Survivors are at the Orange experience level the card generates the amount of Zombies for that card.
In this case it produces two walkers. Under our current system of playing we would place two more walkers at each of the other spawn points for a total of six on the board. This of course is no big deal and the players adjust their planning accordingly and dismiss such a minor threat.
But if we play by the rules the following occurs:
As per the instructions, for Spawn point 2 another card is drawn which produces three runners!
And on the third Spawn point the Orange level card produces two Berserker runners!
Now the entire dynamic of the next turn has changed dramatically as the survivors have much more of a threat to deal with.
So instead of having to deal with just six slow ass walkers, which would be the case as we play now, the Survivors would now have to face, two walkers, three runners and two Berserker runners if we play by the rules.
I believe this fun but oft times difficult game just got a bit more challenging.