Anyone who believes that "everyone" grades on a curve hasn't been to an undergrad college that expect you to learn then. In a lot of them, a 70% would still be a C-.
You have no idea how far in the right Julie has to be for me to agree with her, but she's right.
College tests aren't given to a statistically signficant population to determine a normal distribution.
It's highly unlikely that a professor would possess the superhuman intellect required to come up with a fair distribution simply by having arbitrary 90/80/70 cutoffs.
As such, the possibilities are:
1. They just give grades based on the arbitrary cutoffs. If 90% of the class gets As, so be it. They made the cutoff. If 90% of the class gets Ds, so be it. They didn't make the cutoff to get a higher score. Either result is grossly unfair or patently ridiculous. I can see why less rigorous schools would do this. They just give a lot of As. Makes for happy students.
2. They give a test that results in a normal distribution of scores, then they assign, roughly, a C to the mean or median score and measure out up and down to give the other letter grades.
The odds of a teacher devising a test where the mean score naturally fell at 75% are infinitessimally small.
Julie is right on this one. Though your experience seems to indicate a greater experience with teachers who graded using option 1.