I just did my course-based Master's Degree in the U.S... and while I obviously can only speak based on my experience at one institution, I can assure you that you are looking at your stats in the wrong way doc toothache.
First of all, within both Canada and the US there are hard universities and there are universities where, regardless of the % you need to get an A, it's considered easier. Basically, there is such a wide variation within each country itself that it's dangerous to just compare overall difficulty between the two.
Secondly, the letter grades are more important than percentages. Just because at my Canadian university I needed an 80% in my class to get an "A-", doesn't mean it was easier to get it. At the U.S. university I just graduated from, a "90%" is an A- and in my courses it was just as achievable as it would be in Canada (A- was 80%). I can't remember a single science course I took at McMaster that didn't have a class average in the low 60's (C-)... but I have yet to ever see a professor at Tulane allow a class average to be in the low 60's. It's the letter grades you need to compare. How much of each class = getting A's?
Thirdly, it's not just the lack of dental schools in Canada that is the issue. It's that they take relatively no out-of-province (aka out-of-state/international) students (they are funded by the government!). Thus, if you're like me and you're from the province of Ontario... your only 'realistic' options are the 2 dental schools that are in this province (UofT and UWest). There are
13 provinces/territories in Canada and
Ontario (just 1 of those), houses
over 30% of the Canadian population. So if each dental school in Ontario has 50-60 seats, you can see where the competition comes from. It's a numbers game at the end of the day.
I'm not here to say U.S. schools are not hard (hell, I can only speak from personal experience and I only attended one). I'm not here to say that Canadian schools are not hard. It's a damn spectrum in both countries. The grading scheme for essays, the way multiple choice tests are made, the curves... it's all different. You're not going to automatically get a class where half the students get away with getting an "A-" in Canada just because it is an 80%, that's madness. At least not at the university I attended (maybe at York Uni, j/k).
I'm only one example in this giant spectrum, but my final year of a bio bach. degree in Canada I was able to achieve a 3.85 at McMaster. I'm graduating with a master in cell and molec biology at Tulane with a 3.97 (I was in a program of ~30 with only one other Canadian; the rest were from all over the U.S.). If it's so much easier to get A's in McMaster, why did I excel in my first and only year at Tulane? You could argue that Tulane is an easy school and I could also argue that you don't know which professors I had so that would be a general and inaccurate statement for someone to make.
In conclusion, spread peace and love.
Canadian love.