It floats, so it bulges out from the earth, shifting that mass away from perfect center. Given that it’s not liquid, it bulges out non uniformly in one direction and biases center of mass in one specific direction. It also doesn’t matter too much that it’s not particularly high, as it it covers a large area in the general direction. Anything on the northern hemisphere would bias center of mass in that direction.

EDIT: I’m not totally sure about the bulging out part of the floating ice is true, but there are also glaciers on land such as over Greenland that would shift the center of mass.