I doubt the IRS adjusted anything to match "past history". If the IRS changed your 2020 tax return, it's because they were aware of something you didn't report on the tax return. Could be income you forgot about, or a deduction you were not entitled to, or any number of things.
I myself am self-employed and I just send the IRS 20% of my gross business income each quarter. Then come tax filing time I'm always within $1000 of whatever my tax liability may be, thus no penalties are assessed. Typically, I always get a refund. But I've had two times I actually owed the IRS. Both times it was less than $500 that I owed. Since the amount was less than $1000 no underpayment penalties were assessed. (provided of course, I pay the full amount due by the payment/filing deadline.)
While one can use the feature in the program to figure and manually generate the 1040-ES for quarterly tax payments, I myself find it a waste of time and effort. I have no way possible of even guessing what my earnings will be each quarter. So a few days before the quarterly filing deadline of each quarter, I look at the gross business income for that quarter and then pay online to the IRS 20% of whatever that amount is.
