Why is today the shortest day but the solstice is actually tomorrow? Here in Ashland, Oregon, today (Dec 20, 2020) is the shortest day length of the year at 9:05:45.  But the winter solstice isn't until 2:02 AM PST tomorrow (Dec 21, 2020). Tomorrow's day length is a whopping three seconds longer at 9:05:48.  Why is tomorrow's day length greater than today? It would seem to me that tomorrow's sunrise and sunset are slightly closer to the magic moment of winter solstice and should therefore account for a slightly shorter day.
The other possibility is https://sunrise-sunset.org is inaccurate.

An app that I use, LunaSolCal, show the length of day on Dec 20, 21, and 22 in Ashland OR are 9:05:45, 9:05:44, 9:05:49.
The website https://www.timeanddate.com does not have Ashland OR in its database, so I used nearby Medford OR. The length of day on Dec 20, 21, and 22 are 9:04:50, 9:04:49, and 9:04:53.

Although those two sources have a difference of 1 minute in the length of the day, the pattern is correct and expected: the shortest length of daylight is on the day closest to the winter solstice. Therefore, I think sunrise-sunset.org is off by one day with its calculations.