I didn't understand the difference, so I finally did a search on the two titles, "King in the North" and "King of Winter."
Based on a few of uses of the phrase "King in the North" I had noticed before doing the search, I expected to find that "King in the North" was an incorrect title, and might explain why Robb's reign didn't last long - he never set foot in the north after being crowned, he had been born at Riverrun and just didn't qualify as a true King of the area north of the neck, in