Logic for Comparision Index in Signal Tester

Looking for how the results of the Comparision Index in the Signal Tester is calculated. I got confused when I used the SPY as the comparision index on a test (Jan 2001 through July 2021) in the signal tester and the results showed 100 probablility of gain for the index (SPY). How does signal tester calulate the results…does it just evaluluate the overall index SPY or does it look at all the codes that make up the index? I was expecting it would look at all the codes within the index and thus would be unlikely for a 100% probablity of gain. Thank you so much!

Sorry, I miss-typed the testing period was Jan 2021 to July 2021