WAG is that it might be because Saab had five or six different shocks for the rear. The only difference was the valving that they changed to match the five or six springs they supplied (which changed for body styles and engine/option configuration).
Bottom line, you could ask Sachs. They will probably tell you one is slightly stiffer.
If you have them in hand, stand one on a hard surface. Push down on it until it gently bottoms. Release. Time how long it takes to return to fully extended. Repeat with others. Longest return time is the stiffer offering.