Many games for the HD consoles tend to be overhyped (GTAIV) and receive inflated review scores. So most people buy it week 1. Most of the sales of these games are also to die hard fans of the series who have been waiting years (or in the case of Madden and FIFA 12 months) for the game to be released and are there on day 1 to get the game. But then they play it and discover it wasn't the second coming of Christ they were led to believe it was. So they tell their friends not to bother and many of them sell the game so there are many copies available second hand (which don't count towards sales).
Evergreen games on the other hand have very positive word of mouth and also have another huge advantage. They are on systems that are selling double what their competitors are selling each week so they have a rapidly expanding user base to sell to (of course this second point doesn't explain why Melee was one of the Gamecubes top selling games from the day it was released to the day they stopped making Gamecubes, it just had super positive word of mouth).
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