Sorry, but I couldn't help but laugh- my 3yo DD has FINALLY just STARTED to stop doing that (as much). I think they are hearing what we say, but not processing it fast enough for some reason.
A few months ago, we were coming out of Target together- my husband bought her some play shoes, and she was happily carrying the bag. This is how our conversation went.
me: Did Daddy buy you new shoes?
her: ...........what?
me: Did Daddy buy you new shoes?
her: .......what?
me: Shoes. Did Daddy buy you shoes?
her: no.
me: What did Daddy buy for you?
her: .........shoes
Argh!
I think she was taking in all the sight/sounds of her surroundings, and having a hard time doing that and processing what I was saying.
I found that sometimes I had to get right in front her, make sure she was focusing on me, and use the fewest words needed to convey the message. She's getting close to 3 1/2, and I've noticed she's been doing it a lot less.
Hang in there- I know it's maddening!