It could also be that George and Ringo's capes were actually worn by them, and feature in the film and on the album cover, while the piano - though John and Paul may have played it, and worked on songs between takes - was merely set dressing owned by Twickenham Studios.
In a way, it's more interesting that Ringo's cape sold for more than George's (especially with, being slightly ghoulish, but accepting that that's how it is, the fact that items associated with dead celebrities tend to sell for more than those associated with the living).