A little more info to help fill in the picture. A piano is tuned to itself. Each piano is tuned differently because of the differecne in wood, strings etc.

A well tuned piano will not neccisarily sound in tune with another well tuned piano.

Example, during one of the Olymics around 100 Kimball pianos were set up in the arena. They tuned all the pianos the same, some average setting used on an electronic strobe. All the pianos were in tune with each other, but each piano was out of tune with itself.

http://en.wikipedia.org/wiki/Piano_tuning