You're right. But when I hit Ctrl-P from any page, it defaults to printing in A4. I guess the desktop locale settings are in charge of that.
Canada uses the same inch-based US sizes but denoted in mm (called P sizes, I think - I’m also not Canadian).
Canadian here, we use the US sizes and describe them in inches (or by name, so we say "letter" or "8.5 by 11"). Microsoft Office uses both inches and millimetres but I always thought they did that for outside of Canada too.