What to do if I find actual exam marks higher than the marks I made for my exam?
Options: 
Re-scale the grades by multiplying a student's score, x, by 20/15. Thus a score of 12 out of 15 (which is 80%) would become 16 out of 20 (still 80%).

Just give them five more questions. Did you plan to get more data for grading purposes? You can always get more.