Bribo,

I think the "director' which adjusts the scenario (difficulty) on the fly based on the teams performance may also have something to do with it.

In Left 4 Dead 2, the Director has been improved to encourage more participation by players, forcing players through difficult gauntlets to reach the extraction point. It will also have the ability to alter elements of the level such as placement of walls, level layout, lighting, and weather conditions, making each play session unique. The Director will now reward players for taking longer or more difficult paths through each episode by providing more useful equipment, such as incendiary ammo, along these riskier paths.