Stacy Abrams. Georgia Democrats have probably best ground game in country so others may have been accurate, just too many weren't showing up thus underestimating Democrats. No other state Democrats I know of has anyone with as good a ground game as Stacy Abrams. Florida and Ohio Democrats are in the absolute dumps.
You do realize its Brian Kemp who was the one that registered millions of voters in GA with AVR.
My theory is that Trump's campaign didn't focus on it in terms of registering voters and getting them to the polls. This meant there was less chance of not picking up new/infrequent Trump voters there.
Also, it is a state that is rapidly trending Dem (faster than people expected) in a year where there was a Dem polling bias. These two things probably cancelled each other out a bit.
Georgia is the one state where Trump outspent Biden.