What I'm trying to say is that Forza games don't have legs because MS/Turn 10 likes to release every Forza games every two years or so.
As far as I know, MS likes to bundle Forza now and then. There must be a reason for it. My best guess is lack of sales. Why it gotta be forza? Why can't MS heavily bundled other exclusive titles like Halo or Gear of War. Maybe because Halo/GoW has legs. These games can sell on its own. Forza can't so MS likes to bundle it. Same goes for Alan Wake so MS had too bundled it too.
I don't where you got your figures from because I don't think Forza 3/4 sold that much on its own.
I was looking over the PAL/EUR Charts and Forza titles usually fall off from the charts after 4 months or so. Thats where most of its sale are since racing sims are pretty much dead in the U.S.
NPD for Forza 3/4 launch numbers wasn't that impressive at all. I recalled mostly everybody (even myself) at Neogaf NPD thread was stunned to see Dark Souls outsold Forza 4.
Just to let you know Horizon 2 did not chart at all last week in PAL/EUR Charts. Maybe because MS packed it in over there. What does that tells you? Forza can't sell on its own. Last year, GT6 charted by itself throughout the Holidays and after Xmas.
http://www.neogaf.com/forum/showthread.php?t=942742
http://neogaf.com/forum/showthread.php?t=556166