Generally there are pretty wide swings, but a thing to keep in mind is that Trump is an incumbent and Biden is about as well known as a challenger could possibly be.

So the lack of wild swings isn't really surprising.

If Biden wins -- and obviously we still have two weeks -- I think this will actually wind up recorded as a pretty simple story:

"People have never really liked Donald Trump to begin with -- he's never had a net-approval north of zero -- and Trump then fucked-up the Covid response.  George Floyd was then murdered, and the fact that it was such a simple case and all on video shifted the perception of how black people are treated by cops, and Trump then fucked-up his response to that.  Trump continued to say crazy shit.  Trump continued fucking up the Covid response, despite Covid being the most important issue.  Meanwhile, everybody knows who Joe Biden is, and Joe Biden ran on `Y'all aren't excited by me, and that's fair, but he's a fuck-up and I'll fix his fuck-ups.'  And he mostly kept his mouth shut and let Trump hang himself."

Be nice to America. Or we'll bring democracy to your country.