bad math, though. The BULK of new zombies would have been right at the start of the outbreak. They either turned or killed a huge percentage of the humans. From now on, the new zombie creation would be a trickle at best.
That's the math the producers have cornered themselves into applying, otherwise, there is no zombie threat beyond a few months, as decomposition to the point where tendons, etc, rot away only takes a very short amount of time. Of course, the producers would argue that the laws of nature do not apply in their fantasy world, for the same reason zombies could not exist in the real world.
So for their own ends, large numbers of zombies will continue to be produced forever.