I too was trying to see what the sizes actually are. I wanted to make sure the larger size would fit and I wasn't sure if the range they gave was for both bands or just one band. So I asked Apple. First she explained to me that S/M = Small/Medium and M/L= Medium/Large. Because I guess that wasn't obvious. Then she explained that mm= Millimeters. Because that too somehow needed explanation. Then she proceeded to tell me that MM can convert to inches.... I can't even make this up. Then she measured her's, "I measured mine from one end to the next while it is attached to the case." After some time, and a few rude comments on my end, I finally got this:
WHEW! It was not easy! hope it's helpful.