Just to close the loop - to satisfy my curiosity I contacted Specialized. The story they gave is that if their bike designers can get away with a medium cage, sometimes they'll do it. No problem either way with large or medium cage.

Now that I put the large cage derailleur on along with a new chain, I am finding the rest of my drivetrain is worn - may need new chainrings too, which is par for the course I guess . . . $$$