They say this because they have never sat down and had dinner with Rosie and Lily and never had to endure a conversation with the two of them.
Rosie: Mama, I am going to drink all of my milk because milk makes you big and strong.
Me: That's right Rosie it does. Gosh when you grow up you are going to be so smart just like Lily.
Rosie: I don't want to be smart Mama, I want to be just like you.
Lily: Oh so you'd rather be blonde than smart Rosie huh?
Me: You two are right charmers! I happen to be very smart and just a little blonde.
Lily: Mummy, you are really blonde.
Good thing I haven't got a fragile ego.