(Why) are polar bears more common in Labrador than in southern Greenland?
I think the lack of sea ice coverage would point you somewhere- even with the maximum sea ice coverage picture, the SW corner of Greenland has next to none, which may impact the carrying capacity for both bears and prey animals on that narrow strip of land between the ice cap and the sea- less space and less things to hunt, less ways to get around, etc. And then there's not much continental shelf over there, either.

Labrador is surrounded by more continental shelf, which might contribute to higher levels of sea ice at high levels and greater seasonality. Might lead to more prey?

This article might get you somewhere:Polar Bears in a Warming Climate