This is a simple chart plotting the distribution of Clinton's margin in the September national polls in 2016 (blue) and Biden's in 2020 (red). What should stand out here is a. Biden's lead is clearly bigger (7 vs. 2ish). b. But also those averages comes with a margin of error.

What I mean by that is when you see Biden up say 9 or up 5 in a national poll, those results are perfectly normal and perfectly in-line with the average given a margin of error. If we weren't seeing it, then I'd be greatly worried.

Chances are the average is right... but sometimes, of course, it isn't... Which way might the error go? Who knows. But what we can clearly say is Biden heads into debate 1 in much better position than Clinton did.