I've heard the same thing, but until now, I have not thought of a way to find out.
Here is a possible solution:
Play the game up to any given level (Level 10, for example). When the first zombie comes out, shoot that zombie with a semi-auto rifle and count the number of shots to the upper chest it takes to put him down. End the game and start a new match, but this time...
Play the hide and seek game at the Level 9, and when Level 10 starts, use the same rifle when the next round starts and count how many rounds it takes to put the first zombie down (using chest shots), then end the game.
If all possible factors are the same, then this controlled experiment should reveal some reliable results...especially if you take the time to kill at least 5 zombies and do this several times. Nothing becomes fact until it can be proven time and time again.