Annette was a better dancer than Stephanie
I like the character Stephanie. She was a tough and helped Tony see a life beyond the disco and paint store.
But she was nothing special as a dancer. I blame the producers. Obviously they spent their entire dance training budget on Travolta, when it was necessary to have the character Stephanie be a good dancer too.
Annette was a decent dancer in the first disco scene. Maybe they should have switched the casting of Annette and Stephanie.