Unlike in high school where the grade percentages are pre determined. ie a 90% is the cutoff for an A and it is expected that students will score that or higher, College professors see that as making the test too easy with the potential of having too many students get A's. Instead they will make the test longer or harder such that the best grade is 60/100 and the average grade is around 40. The will then scale things such that the top 10% get A, 20% B etc.

Sometime the prof screws up and the test is too hard where the aveage grade is 30. Sometimes some over-achieving git 'busts the curve' and scores a 95 and the next highest grade is a 60 - making it hard for the prof to justify giving A's to the top 10%

That sounds like what it is from my experience as well but it's really up to professor.

One exam was really hard such that your new grade was sqrt(#correct/#total). Another was to make new total the value of highest correct score, e.g. everybody in class got 0-20 out of 100, new total is 20.