You are ignoring some things here:
-The Gamecube was alive for a lot more time than Dreamcast, so its logic it has a lower average monthly sales figure.
-The Wii was released just 9 (9 right?) months ago, and the PS2 had its pike on monthly sales when it got really cheap... around $175 if I remember correctly, so is premature to state that the Wii can't reach the PS2 in sales, while it hasn't seen its first price cut yet.
-Don't forget that some people that got its PS2 to die, got a new one, and to my knowledge is a rather high amount, considering the total sales of the PS2... for example 10% would be around 12 millions.
Just wanted to point that out.