What's the difference between an American and a Canadian?
Q: What's the difference between an American and a Canadian?
A: A Canadian not only has a sense of humour but can also spell it.
This is another Canadian joke. It is an insult against Americans. The humor is that Americans spell the word humor differently than Canadians do. This joke could reasonably be told by Canadians or people from England. Both countries spell things differently than Americans do.
Another part of the insult is that Americans do not have a sense of humor. I personally have not noticed this, however since I am an American, perhaps I am blind to this failure in all Americans.
Personally, if I were trying to insult Americans in a joke, I would have chosen another point to make fun of them. There are quite a few of them to choose from, and the spelling of the word humor is not on the top of my list when it comes to funny ways to insult people.