It's actually an easier mistake to make than you think. I had the exact same thing happen to me last year when booking a flight. I went to aircanada.com (from home in Canada) like I normally do, but it automatically directed me to the U.S. website, and I didn't notice. Yes, there is a small icon at the top right, but I didn't notice it at the time.

On the screen where you actually select the flights, there's no indication of currency. And when you go to finalize the payment, it does say "Grand Total - Canadian dollars" on the text over at the left, but there's no CAD or anything like that adjacent to the currency amounts, and your eyes tend to gravitate towards the $ breakdown on the right, not the jumble of text over on the left.

I'd noticed it directly after I booked when I was reviewing the receipt email, and called up AC within a few minutes. Because it was within 24 hours, they would have refunded the fare, but I'd get hit with the credit card FX charges twice, so I just stuck with the booking in US$. In the end, it probably cost me about $80 extra in credit card FX charges due to paying in USD. And yes, I didn't get as good a deal as I thought I was getting due to the US vs. CDN dollars, but really the only difference where I was out $ was the foreign transaction fees.

I didn't do anything different that time, but for some reason, it automatically sent me to the U.S. website without any type of prompting/notification. Unless you happen to notice the icon at the top right, it's a fairly easy mistake to make.