Why do Vanguard Mutual funds have slightly better performance than their ETF counterparts?

I’m trying to understand why in general mutual funds tend to very slightly outperform their ETF counterparts. I’ve put an example here but it seems to be true for the majority of mutual fund / ETFs I’ve seen.

I checked the expense ratios and the portfolio compositions and they’re usually the same, so it’s strange that there’s a slight positive difference in mutual funds.

For example, the ETF VGT and its counterpart, VITAX.

Thanks for any insights.