As I understood the directions, if Mama et Papa spoke French back home in Canada, then the kids' entry would be "Canada-French" no matter what province they lived in. Everyone else gets recorded as "Canada-Eng", even if they spoke Hungarian around the house, according to the instructions for 1930.
So, Pierre la Blatte would be enumerated as C/F even if he lived in Manitoba, and Jack Moosehead would be enumerated as C/E even if he lived in Quebec.
While most of the French-speaking people in Canada live in Quebec, they move around, just like Americans do; you can find Yankee twangs in Texas and Texas drawls in Maine, if you look around.
There may have been a subtle anti-Catholic bias to the distinction, since they couldn't ask about religion. Most French Canadians are Catholic, most English ones are Protestant.