The professor graded on a curve, which is pretty common in college. If he hadn’t, almost everyone would have failed the class, and the professor would have looked like he didn’t know how to teach.
There’s no definitive standard about how grades are given out in US universities. A lot of my classes were graded on a curve, where maybe the top 15% in the class got an A, the next 20% got a B, the next 30% got a C, the next 20% got a D, and the bottom 15% got an F. Other classes had strict ranges, for example, 90%-100% was an A, 80%-89% was a B, and so on.
If the average for the first exam was 61%, then realistically that’s probably going to be a C rather than a D-.