Huh? That doesn't seem strange at all. It's the first place I would have guessed - based on it being really extreme, really big and really cold.

"Cold" isn't typically associated with "dry" in most people's mental maps, as rain tends to be cold, and snow is very cold, and even the most commonly encountered form of ice (icecubes) melts quick enough too; and therefore generally most of everyday coldness gets anti-associated with dryness.

Ofcourse Antarctica is not everyday coldness - the ice in most of Antarctica is very far from temperatures that would make it liquid... But I understand how it could surprise someone who hadn't thought it through.