Lunch has more or less turned into a common name for a "meal" around noon, usually between 11:00 and 13:00. A lot of places, they are serving warm lunch, which is pretty common in Sweden, and it's getting more usual to eat a full meal, altså måltid, at that time of the day :) But others simply having their matpakke, fruit or salad. Or nothing at all :) :) :)
So, both a full meal and just a snack is referred to as "lunch"? Do they still call it "lunch" if the snack is eaten later in the day or evening? That is strange. I would use "lunsj" when describing a regular meal (small-ish or full, but not a small snack) eaten after breakfast and before dinner.
As far as I can tell, they think of it like a snack--just a really big snack. And they call it "lunch".
It sounds like this could be a combination of Norwegian "lunsj" (as you said,, any meal between breakfast and dinner) and farmers. In the old days (before combines and tractors with GPS), farming was hard, physical labor and farmers needed to eat big meals. During the harvest, especially, they might have a very late dinner (so they can get more work done) but several "lunches" during the day.