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.