I think there is a lot more to this deal than meets the eye.
I think probably that Button would have settled for the money that Brawn were offering, but he wanted a multi-year deal, and Mercedes wouldn't play ball, because after 2010 they want young Mr vettel in the team, along with either Heidfeld or Rosberg.
If say McLaren offered him £6m x 3 =£18m, as opposed to £6m x 1=£6m.
I reckon that's the reason. Why else would Jenson willingly relinguish top-dog position, and put himself up against Hamilton? By the way, I think Lewis has done enough to shut up the idiots who thought he only won the championship because he had a McLaren in 2008.