Actually, its far more the fact that Nintendo has a few titles that end up having a very long lifespan, whereas the 360 titles end up doing more of it front-loaded wise. Its not really a hardcore thing.
Nintendo has the advantage of WarioWare, Wii Play, and Super Paper Mario, which have been able (expecially Wii Play) to sell fairly well over a long period of time. In the case of Wii Play, it sells around 50k per week, helping the overall scheme of things.
Also, you have Wii Sports added into that chart, making it harder, as if the 360 had an extra 150,000 software units a week (like the Wii does from Wii Sports/Play), it'd be far more even.
Once the Wii has major software coming out often, you'll see it similar. For refference go back to my Japanese chart with the same figures. The Wii is doing the exact same thing the 360 is in the US, due to the fact no games are bundled.
Again, it also comes down to what software is being presented. When the 360 has shooter-heavy fare, that does cater to the more hardcore, sales do spike, as GRAW 2, GH2, Crackdown, and Lost Planet all sold 200k+ units their first week. Any game that sells THAT high will end up dropping very quickly the next week. The Wii hasn't had that advantage of having any sort of major software (since Zelda:TP) to move 200k+ copies in a week.
Also, within the past....6 weeks, with the 360 lacking shooters, it's sales have been far more consistant, as sales have been mainly pushed by Command & Conquer and Spiderman, which are mid-road favorites, as well as tons of GH2 sales.
Back from the dead, I'm afraid.
