No it isn't how the "American grading system works." There is no such thing as an "American Grading System." I've only had one class that graded on a true bell curve, and that was because the class had something like 200 people in it, making such a curve fairly accurate. I attend a private university in the top 25 of U.S Universities, and the grades usually are deflated, not inflated (a C here is like a B at Community College in terms of preparation.) Most classes that grade on a curve (and they are probably like 25% of my classes at that) will only boost your score, not take away (or if they do take away, it is by a little bit/it is bounded.) But in the end, it is entirely up to the professor how grading works. 

An example of a typical curve is the following (from my first semester Honor's Physics class):

Take the square root of your score. Add the number of points on to your score. Wherever your score fell, cutoff wise, is your new grade. So if you got a 30% the square root of that would be 5.47. So your new percentage is 35.47%. At 70% the square root is 8.3%. So your new score is 78.3% (which could boost you from a C to a B grade depending on the course/exam.) So in the end it helps the people who did better, and helps less the people who did worse. Most professors usually choose curves like this. That way they can measure the actual success of the student's knowledge of the content, without hurting all students by giving them overly hard exams (sometimes they'll just adjust the cut-offs as well and grade without a curve.) I've had cut-offs where 30% was a C, 30-50% a B, and 50%+ an A. 

In K-12, the only time I experienced curved scores were in my AP classes (college level courses offered in high school), and again it wasn't a true bell curve. Basically the lowest score you could get in those classes was a C and the highest an A. 

I think I wrote my comment sometime very early in the morning lol. You and SC are right, not every course/test is graded by the bell curve. I should not have generalized. I am in Canada and I have been graded by the bell curve a few times. My brother has too, which gives me my best example. He had a Psychology course in which the Professor needed a class average between 70-72 (I think those are the correct numbers) due to some regulation put forward by the Canadian Psychological Association (I should text him later to make sure I am not incorrect in my statement)

I don't think its the review score that has been harsher, I think its the games that are just mediocre in general. Seems like they ran out of ideas or are just playing it safe.
I have yet to play a game that was not deserving of its reviews, except for GTA4 which I think was a bit high. Some will say well look at Nintendo games they have high scores and yet its the same game, those people usually didn't play those games as they are fantastic and Nintendo brings new ideas in its design or the way we play them and each new game feels fresh.