I wonder if it's because she was raised to be OK with humans and/or birds. Alex had a "special" childhood and I'd expect her to be mainly socialized to humans, since she didn't have much parent contact as a baby. I don't remember what Georgie's background is like (if I ever knew it at all) but it sounds like he's mainly socialized to birds.
But Lucy can go both ways.

She enjoys you when you're available and enjoys the other birds when you're not.