Interesting, thanks to you both for posting the links.
My first thought was perhaps the polar bear population was decreasing and allowing the grizzly to move in. So I found it surprising in the article when it mentioned grizzly bears killing polar bears (but no mention of the other way around). A little googling and it does appear to be the case that even though a polar bear is bigger the grizzly is tougher.
Having worked in Alaska's arctic for a quarter century, I only witnessed polar bears and grizzly in the same location once. The grizzly chased the polar bears off the island. No fighting was observed as the white bears departed quickly and willingly.