As has been proved numerous times, the Star Wars continuity is not set in stone. Ahsoka probably was not even an idea when they made RotS. Now she is great for marketing and getting people involved in Star Wars. I have met several little boys who believe that they are the most hard-core fans in the world...and all they've ever seen are the Clone Wars. Kind of annoys me, actually, but they will eventually consume more of the universe and become the well-rounded kind of hard-core fan.