This was simply because of the rhythm of the music. Do you remember Mel ever having to snap his fingers a la 1 2 3, to complete a sentence. I saw at least one example of this during the talk show days. He might have been on The Tonight Show.Could never figure out why he would always stutter when he talked, but when he sang, his voice was great!!
This may also be true.Mr Tillis stated in a interview that you use one side of the brain to talk and the other side to sing.