Actually, the Turbo S was only in 88, the 89-91 951s although identical to the 88 TS, were not considered S's. It all comes from the Porsche tradition of offering preproduction technology in a limited run of cars.
In late 87, they already had all of the updates for the 951 worked out, but were not ready to do full scale production of the reworked 951, so they made a thousand S models, with the 245 hp, K26/#8, LSD, M030... The things that defined the 88S as seperate from the 88 951 were all standard in the 89-91 951, and they dropped the S designation.
Another useless bit of Porsche trivia!
http://vista.pca.org/stl/944turbo.htm