The planetary sets in the autos are odd beasts. They can make a lot of different gear ratios with very few parts. Heck, if I wanted to I could go rip my old Chevy 350 hydromatic (A 3 speed, direct drive transmission) out of my Olds, tweak the valve body a bit, and get a 9 speed trans. Granted it would have a totally useless triple OD reverse range, but it's still a 9 speed trans with only 2 gear sets.
A simple planetary can produce Reverse, Reverse OD, under drive, direct drive and Over Drive, with just that one gear set.