Probably because Sony hasn't made much noise about the PS4 and still has big exclusives new IP slated to release in 2013 ( Last of US, Beyond).
Others console are starting to suffer from the incoming next gen factor where some customers don't want to purchase a new console with an upgrade coming now ( Wii U) or next year... Even if in practice the PS4 will probably release around the same time as the Xbox720
PS3-Xbox360 gap : 1.5 millions and going up in PS3 favor !
PS3-Wii gap : 20 millions and going down !
