How does IVV go down and the other one goes up I don't think I like the IVv so much

I’ve been holding both IVV and SPY, and I noticed something odd today. My IVV position (50 shares, ~$33.5k) actually showed a small drop of about $4.50, while SPY (183 shares, ~$122k) was up over $113 on the same day. Both are supposed to track the S&P 500, so why would one go red while the other is green? I understand they’re different funds from different issuers, but the tracking should be nearly identical. Does anyone else see this kind of divergence, and is there a real reason to prefer one over the other long term?