Many states report absentee and postal votes at a different time to election day votes. Typically votes on Election day are recorded and counted electronically. Postal votes are counted by hand after the Election Day votes have been counted.
Most years this allows for a rapid calling of the winner, as the number of postal votes is small in comparison to Election day votes. This was not the case in 2020.
Moreover postal votes trended blue, again 538 noted:
Democrats are much likelier than Republicans to say they will vote by mail — which makes sense given that Democrats also tend to be more supportive of mail voting. (By contrast, the Republican standard bearer, President Trump, has repeatedly and inaccurately assailed mail voting as ripe for fraud.)
So in states that counted postal votes later, a "blue shift" was seen. In States that counted postal votes first there would be a "Red shift". For example in Ohio, the early votes were counted, then Election day votes and finally late arriving postal votes. So first Biden would have an advantage, then Trump would come back, and lastly Biden would gain again.
The shifts were forecast in each state, and you can compare this to the actual results to test or falsify this model.