What am I missing?

Matthew is learning new words everyday and sometimes, his pronunciation is astonishingly good. Sometimes he'll say a word and I'll be surprised how clear it is. He clearly is able to listen and imitate.

But here's our conversation this morning which has me thinking I'm just missing something in the way he is processing things:

Me: Matthew can you say 'airplane'?
Matthew: "airplane."
Me: Good! Matthew can you say 'car'?
Matthew: "Car!"
Me: Can you say 'Michael'
Matthew: "Michael."
Me: Can you say 'bus'?
Matthew: "Bus!"
Me: Can you say 'Choo-Choo-Train?'
Matthew: "Choo-Choo Train"
Me: Good! Can you say helicopter?

Matthew: "Stee!"