why the puzzle piece works for me

I'm puzzeled by Ben.
I was reading an expert from a Dr. I forget his name, but he talks about how difficult it is for autistic kids/people to communicate. And I believe that.
But why is it so easy for Ben to communicate some things?
If I sing, he's VERY able to let me know he doesn't want me to (Everyone's a critic)
If I ask if he wants to go swimming, he's able to communicate an emphatic yes.
Everyday he knows to ask for the cookie I'm packing when I pick him up from daycare,
yet.. everyday, he doesn't get "Bye Mommie" right.
What is the key, or the missing puzzle piece that let's some things be so easily communicated, clearly!, though typically in an elementry way, while others seem so COMPLETELY out of reach.
Is it points of interest? Some things get through while others stay way on the outside?
I'm puzzled!