Steinway is constantly improving their pianos in the small details, as well as their production techniques. And like PRS, they have limited editions and special runs, plus their version of a Private Stock piano called "Special Collection." I'm sure other makers are, too. Being very large instruments, the tone variations from piano to piano of the same model are more noticeable than between two guitars of the same model, so their main concern is to have consistency, no duds, etc. You don't want a $100,000 grand piano sitting at a dealer for years and years because it doesn't sound very good compared to others of the same model. Piano makers are also obsessed with improving the action and responsiveness, as the keyboard mechanisms on a grand piano have a crapload of parts, all of which affect the playability and tone of the instrument. Even the density of the felt hammers affects the tone.