It is not 10 squared but 2 to the 10th, which is 1024! This is the number of different subsets of a set with 10 elements and these correspond to all the different choices (or combinations).

If you are going to build a subset of the numbers 1,2,3,...,10 then you have to look at them, one at a time, and for each, decide if you are going to include it or not. Thus for each of the numbers 1,2,3,...,10 you have two choices, include it or don't include it. Hence the number of choices you have altogether is

222...2 = 2¹⁰