"If so, why does BS say DD to begin with?"
How is a 9 v 2 composed? 9 can be either 7,2, 6,3, 4,5. 2 is composed of one 2.
So aside from the 7,2 v 2, which is a +2 RC, you have a +3 RC from 63 v 2 or 45 v 2. With 2 decks that is 1.5 TC. So that is why in double deck 9 v 2 is a basic...