Replying to Whacked test grading

So I had a test in differential equations that I bombed.  It was 8 problems.  I didn't know the first 2 at all and the others were either correct or at the most 50% correct.  After getting the answer key, I realized that there was no way to get above a 50%.   Well I finally got it back and I some how got an 82.  I got almost full credit on the first 2 problems without any of the correct work and i got crazy partial credit for the other ones.  On one hand, I was pumped to do so much better than I expected, but on the other, I feel bad because I definitely didn't deserve anything more than 50%.  This happen to anyone else?  Obviously, I'm not going to get it changed or anything, I just feel a little bad.