Mail ballots come from different counties with highly different demographics. You get massive Biden support in one city and massive Trump support in another rur

Mail ballots come from different counties with highly different demographics. You get massive Biden support in one city and massive Trump support in another rural county - it's not surprising at all. There's also differences in when the ballots arrived: some states could process ballots ahead of time so the late counted ones are only those that arrived late and may be more or less D/R leaning. And different states have different voter registration policies; that's why Arizona has had relatively good Trump performance on the later counts since there are a large number of Republican voters using vote-by-mail and there always have been in Arizona. There is boundless discussion of the specifics of this on twitter, etc. I don't think swing state/non-swing state is a useful categorization here. Just look at the where the ballots are coming from directly.