I have recently noticed that yahoo finance and google finance provide different closing prices on some stocks. For example, the previous days closing price as of 10/9/2019 for DAL is listed as $53.20 at https://finance.yahoo.com/quote/DAL?p=DAL&.tsrc=fin-srch. However, according to google finance the previous day's closing price is $53.18. Sometimes however the two prices match, but sometimes they are different. My question is why is that? I was expecting them to always be the same.