I remember hearing 40% bandied about as the percentage of Wiis which are actually connected to the internet. Then you can add in the fact that some people will play the campaign before they go online, while others might only play the campaign and hate online multiplayer, etc.
Furthermore, almost no Wii game has explosive launch sales. A game which saw more preview videos released on Youtube than on Gametrailers certainly isn't going to jump out of the gates. I know we want to be able to compare 1st week Wii game sales with 1st week HD console game sales, but the patterns are dramatically and consistently different. We see it time and time again: You just can't judge the performance of a Wii game by 1st week sales.
"The worst part about these reviews is they are [subjective]--and their scores often depend on how drunk you got the media at a Street Fighter event." — Mona Hamilton, Capcom Senior VP of Marketing
*Image indefinitely borrowed from BrainBoxLtd without his consent.