Help me understand ETF vs Mutual fund quotes (S&P500)

Hi everyone, I'm rebalancing some of my 401k and was curious why S&P 500 ETF's like VOO are up like 3% today but FXAIX, which is Fidelity's 500 index, is down over 1%? Why is there a 4% net difference here? Is it because mutual funds only sell once a day and at when stocks close for the day the price will change? When my 401k rebalances tonight will it be buying at the -1% or the +3%?

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Thanks!