I just checked a major web site offering guns for sale and had a quick look at every M21 skeet gun---only one M21 had WS1/WS1 choking. So I have answered by own question---this choke combination is uncommon---why shooters chose WS1/WS2 vs WS1/WS1 is open for debate.
Maybe for doubles trap, since you are shooting from approximately the same distance on each shot? Or maybe for skeet for the same reason. I use my Model 21 for skeet that has been opened to Imp/Cyl and Imp/Cyl. It has always worked fairly well. I guess you'd have to ask someone who ordered a gun in that configuration.
