Seagate's datasheet is in error. There are actually 3 flavours of the ST2000DM001. They could have either two 1TB platters or three 667GB platters, and 4,5, or 6 heads.
See the firmware update matrix:http://www.users.on.net/
Also see the latest Product Manual:http://www.seagate.com/files/staticfiles/support/docs/manual/desktop/Barracuda%207200.14/100686584.pdf
Sections 2.1 and 2.7 refer to 3-disk and 2-disk versions of the 2TB model.
Also see this forum thread where the speeds are compared:http://forums.seagate.com/stx/board/crawl_message?board.id=ata_drives&message.id=28513
The 3-disk version is about 20% slower than the 2-disk version.