My brother came up with a good theory. During the 2016 election Hillary barely campaigned obviously because she thought the election was in the bag for her. The plan was democrats would estimate how many fake ballots they would need to beat Trump but they under-estimated his popularity and he ended up beating Hillary.
By 2020 democrats learned their lesson: They would no longer estimate the amount of ballots they needed - They would get an exact amount first either late at night or at some extended time during the week. This is why we saw Biden votes shoot up late at night and why eventually he got such a ludicrously high vote total of 81 million because Trump was so popular at 75 million. If Trump got 100 million votes Biden would have gotten 120 million and so on. Biden getting 81 million votes is as absurd as him getting 500 billion votes
exactly