Backtesting today's S&P 500 members from 2015, an equal-weight universe returned 16.14% a year. Holding each stock only while it was actually in the index returned 11.06%. About a third of the apparent return came from knowing, in 2015, which companies would still be there.
The bias was shared by strategy and benchmark, so active comparisons held up while absolute levels did not. That is why the Helix pages quote no absolute backtest returns.