He is dressed in normal clothes for any kniht, trying to go unnoticed. So it makes sense that you don't recognize him as your king.

That's why you treat him like any other customer when he passes by your market stall. You're polite and friendly, but you don't use proper courtesy when referring to the king.

And that is something that is refreshing for Arthur, to be able to be just him and not the title of king that everyone sees.