Not all universities include the class average on their transcripts, so how would medical schools be able to evaluate the difficulty in that fashion?
I would assume (although I may be wrong) that they look at the level of the course (ie. first, second, third, fourth, etc.) and what a typical course load is for your program. For example, in engineering we had a couple of semesters where we had to take 6 full courses per semester (instead of the 'regular' 5 for standard arts and science programs), so I assume schools that look at program/course difficulty would take that into account. They might also look at the credit weight of the course. But I honestly don't know how they evaluate difficulty!