One of the main problems is that some people look at top 360 game releases and expect PS3 games to sell similar numbers. The PS3 and 360 owners have a different buying pattern, demographics and install base. The PS3 has better launches of some games, while 360 sells more of others. Shooters have much higher launch numbers on the 360, than they do the PS3.
It makes much more sense to base your Killzone2 sales expectations (and future PS3 sales expectations) on other similar PS3 releases. When you do this, it makes the top 5 launch weeks and your expectations will be much more aligned with reality.