ELI5: Why would an ETF like VOO or SPY outperform the S&P500, if even for a single day?

I get the difference between indexes and ETFs, and understand things like small tracking errors, fees, tax implications of dividends, etc... But as of right now 11am EST on June 29, VOO and SPY are up a bit over 1% while the S&P500 index is tracking at about 0.60%. This isn't just a calculation quirk due to dividends being paid. EDIT: I don't think I can attach pictures, but as of 11:32am, SPX is up 0.74%, VOO is up 1.19%. submitted by /u/Boollish [link] [comments]