In my experience, marking in education (especially in exams) is a massive bunch of bullshit. You'll often find yourself getting a grade way off of what you were expecting (higher or lower, it cuts both ways), and that's simply because most teachers / markers are exactly like the rest of us: they'll put in the bare minimum effort required when skim reading over your work and then pull a grade out of their ass.
You should just take it and move on, because for certain you will get fucked over at some other time in some other subject and end up with a lower grade than your work deserves.
In 6th grade I didn't do a science assignment and turned it in late, which would only be 50% credit. The teacher gave me full credit thinking I turned it in on time, and I know this because he would write late on each paper that was obviously late. I told him that the assignment was late I wanted to get the credit that I deserved. He appreciated my honesty and gave me full credit because I showed integrity.
Similar situations have occurred throughout my academic career and while the result weren't always as positive as this one, I'm still glad I went with the correct choice.
I go to one of the most competitive schools in the country and am enrolled in one of the most competitive programs at my school and am looking to get into medical school which is highly, highly competitive. I have to fight and claw my way up the curve just to pass most of my classes and I praise the heavens for every decent grade I manage to get. Two years ago, I might have hesitated accepting a grade I didn't deserve. Now? I wouldn't even blink.
I'm sure your professor knows that. They probably gave you that grade because they like you or they saw you worked hard or they're just nice. I've had professors give me grades that I clearly should not gotten in the past. And even if it is their mistake, of course you shouldn't say anything. Are you nuts?