She talks in babyish language around them. I am sure this is the same story for so many on here. Someone asked if she likes playing with her agemates and she does since she can enjoy just being silly and rowdy and goofy. You really wouldn't think she were a smart kid if you saw her most of the time jumping and making weird animal noises and doing crazy dances so she isn't always looking for some profound mental stimulation from her peers--she goes to adults for so much.
My ds does this as well. Put him in a group of same-age kids and he seems to act sillier and younger than they do. Put him with a group of middle or high schoolers, and he acts just like them! I've never understood that completely, but I think it is his way of trying to fit in with the younger children. He always says he prefers older kids though.