Thought this was a wild extrapolation based on an odd intepretation of some internet rumours? (see this concise explanation).
Basically, it seems that MP3 is the #1 selling game in the US this week, knocking Bioshock off the top spot. That's quite an achievement for the supposedly 'casual' Wii.
Some people, bizarrely, have interpreted the story as saying that MP3 sold more this week than Bioshock sold last week (that's where the 400K comes in). There's nothing in the original Gamasutra story that actually states this though. It's just an odd take on a slightly ambiguous sentence.
Here's the key sentence from the original Gamasutra story.
Metroid Prime 3: Corruption bests former overall sales leader BioShock in its first week of release, becoming the week's single best-selling software title across all platforms in the United States.
I've emboldened the key word here. To me its refers to MP3, not Bioshock. In other words, MP3, in its first week on sale, has sold more than Bioshock this week. That does not mean it sold more than Bioshock did last week (in Bioshock's first week on sale).
"This man has advanced communist views ... He dresses in a bohemian fashion both at his office and in his leisure hours."