If you had 2 or more horses with the same power, what would their difference be? How would they appear in your set() before getting sorted?
len(ss) is going to tell you how many horses there are in the set you have converted to a sorted list. But it doesn't tell you how many horses there are with the same value, if any.
Try creating a custom test case with a list of horses like 5, 6,8,6,3,1. What should the answer be?