Why does it say that your stimulus check is an advance of your 2020 tax refund?

[ProfessorGAC]

11. This

And the taxable portion is only based on the delta between what one made in 2019 & 2020.
As I told Hoyt yesterday, it's not very likely this will affect many people much.
Nobody is interviewing, so hiring to positions that would result in a big wage increase is unlikely, especially since the new position is unlikely to start until July, perhaps later!
So, while those provisions are there, hardly anybody who qualified for the payout is likely to owe anything. Maybe the single person who went from $73,000 to $75,500. The incremental payback on $500 is a pittance for anybody who had no job disruption.