Halo3 has nothing on the randomness in L4D.
I was in a game the other day where my team got a total of 4 tanks (1, 1, 2) over 5 maps. The other team got 6 tanks (1, 1, 1, 1, 2), meaning on average they went further with more players.
It'd be like in any FPS randomly spawning vehicles for...