How do professors interpret normally distributed midterm or final exam scores?
Here is how I normally assign grades in a class of 30-50 students, given a distribution of scores:


Declare the average to be either a B or B+, depending on how satisfied I am with the class's performance as a whole.
To find the A/B cutoff, look above the average for obvious clumps. If, say, the average is 75% and there are ten scores of 85% or higher, and no scores between 80 and 85, then the scores higher than 85 are the As.
There will not always be obvious clumps, and in that case I put the cutoff somewhere between the average and the maximum in a semi-arbitrary way. I may look back at some exams to see if particular students did what I consider A work.
Determine the B/C and C/D cutoffs, then pluses and minuses, using a similar process.
To distinguish Ds from Fs, I don't make a numerical cutoff. Fs are reserved for students who have not shown me that they're actually trying.


The benefit of this method is that if I accidentally make the exam too hard or too long, the students don't pay the price. But I always try to make my exams straightforward, with at most one challenging problem, because experience shows that asking students to regurgitate what I've told them already provides a good enough range of scores to separate students from each other. If I've done things right, the average is usually in the low 80s.

Now, your professor may or may not use a system like mine. He may simply enter the scores into the grade book and use rigid, pre-determined cutoffs, regardless of the class's performance. The only way to know is to ask him.

By the way, you've observed that the people who spent more time studying got higher scores, and this strikes me as completely fair. If the class could be aced without studying, it wouldn't be a very useful class.