The shopkeeper has just asked you if you are an American. He cannot tell the difference in Canadian and American accents. Americans are not popular in Britain. Their pay is very high compared to British soldiers, sailors or workers so they get a lot of attention from the local girls. Americans sometimes aren't very appreciative of British ways and how much the British people have already given up for the war effort.

Your response led the shop keeper to think you are an American. He tries to "pull a fast one" with his prices and he acts rudely to you and your buddies. Later on, a local explains the situation to you and the locals give you much better treatment.