My 401k contributions through Fidelity go to the FID 500 INDEX. I have personal investments in SPY. Today was a good rally for SPY, but FID 500 INDEX showing -4%. Am I missing something?

I changed my 401k contributions to the FID 500 Index about 8 years ago assuming it was the closest I could get to an S&P 500 Index.

Today I checked my account just because today was a rally, and they don't even match closely at all. Am I missing something?