Why is the count of unique results from rolling two dice once 7C2?

Question

By "unique results" I mean that the order of the two resulting numbers does not matter - a 1 and a 2 is the same as a 2 and a 1. I have used a spreadsheet to brute force the answer of 21, and inspection of the spreadsheet makes it clear why this is the correct answer. However, 21 = 7C2; given the nature of the problem, I cannot believe this is a coincidence, but for the life of me I cannot figure out where the 7 comes from when you are rolling two 6-sided dice. Thanks in advance for shining light on my darkness...