I think Forza 2 is a difficult one. Its unfortunate that Vgchartz does not seperate the bundles from the unbundled games.
This generation shows a trend, especially with the HD consoles of increasing sales for franchises between generations. I don't think this has anything to do with a change in userbases, but more to do with fewer big releases being made and the resultant higher proportion of sales these titles have. With less competition from big titles, the titles which do release tend to do better than you would expect. I don't think this is important for the titles like GTA IV, but more important for titles which aren't released every year in the 750k -> 1.5M bracket.
Tease.