The difference is that Resistance and Motorstorm were heavily bundled AND were lunch titles, thus giving them higher sells than what they would've get normally.
On the other hand, Uncharted sold while there was a lot more competition, and sold mostly because of word of mouth, hence it's a-lot-bigger-than-normal legs, so it's fanbase is bigger than Resistance/Motorstorm.
Add the fact that the first didn't have multiplayer, so a lot of pepole didn't buy it but rented it, and that a lot of pepole sold it, and you can see that Uncharted 2 will probably sell a lot more than the first.
Bet with Dr.A.Peter.Nintendo that Super Mario Galaxy 2 won't sell 15 million copies up to six months after it's release, the winner will get Avatar control for a week and signature control for a month.