It is! It's happening right now in NY and CA, which (being big, lazy, and mostly single-party) count their ballots like mollases. Biden's lead there has been steadily and inexorably climbing. And this despite the fact that they are neither swing states nor Virginia.

Can I flip the question around: can you show me a state with late mail ballot counting that is not trending toward Biden? The only exception I can think of is AZ over the past few days, but they're processing primarily provisionals in red counties at this point. The blue counties are done.

Edit: or alternatively, why is Arizona not evidence of fraud by Trump, given that it counted up very rapidly to Biden, leveled off when the early vote was complete, and then began processing different ballots that skewed differently. CLEARLY Trump is cheating with those provisionals, right?

The whole thesis is just silly. Different counties count at different rates. Different populations vote by different mechanisms, and the reported vote over time is an essentially random walk within that space. There's no mechanism by which you'd expect a consistent rate of margin growth. It doesn't even make sense.