Even as the Arctic has become less icy, the Antarctic has seen more sea ice.
Scientists are still noodling it out. But a couple theories are gaining traction.
First, the southern hole in the ozone serves as a bit of a heat vent, allowing for cooler temperatures at that end of the globe even as the build-up of carbon dioxide and other greenhouse gases appears to be warming the Earth.
Next, ocean circulation may be changing. The waters of the ocean could be more stratified, meaning the warmer water below is mixing less with colder water on the surface. That means more ice.
Finally, it might have to do with how water logged the sea ice is. I'm a little shaky on this point, but it has to do with snow becoming wet and turning to ice.
Try reading this piece for a longer explanation.