Snow?
I would guess that the Antarctic is growing because of more snow. We know that warmer temperatures mean more moisture in the air & it might mean that the moisture reaches deeper into the Antarctic where it can accumulate & expand the glacial ice sheets. My guess is the current models expected the excess moisture to still be frozen & fall into the ocean or fringe ice prior to reaching the glacial zone. During prior warming periods of the Earth, there was not a continent located over one of the poles & continents handle precipitation very differently from oceans.
Let's hope that the Earth remains more resilient to excess heat & CO2 than we thought. Expansion of the Antarctic ice does a lot to negate the melting of the Arctic.