When zombies makes a noise the sound gets "stuck" in the speaker you first heard it in.
An example: You first hear a zombie in your left speaker, you turn to face the zombie, so the sound should now play in both speakers but instead it is still playing in the left speaker fooling you to think that you have more zombies on your left. The sound even continues to play after the zombie is dead.

You can see this in Hicks recently released video on the enfusion engine. https://youtu.be/eFhTsoM-KZI?t=5m39s You can clearly see that he gets fooled by this sound bug and makes a complete 360 spin trying to track a zombie that he has already killed.