Why do folk say that, it should be "Yes, I agree with you"
I notice lots of people do this, including "our" Mr Fry (Shock horror)
One even hears "No, Yes, No you are right", (often in TV interviews etc).
I reckon it is an adversarial or perhaps a superiority thing, "I don't want to be seen to agree with YOU, although I actually do agree".
As Stephen is "guilty" of using this strange terminology I would love to hear thoughts on where it comes from.
John B in the N.W. UK