As it has been mentioned a couple of times, Norway and Denmark (unlike sweden) is members of NATO, and a charter within NATO requires all member countries to provide airlifts in case of a war. SAS will of course provide a large percentage of the Denmark and Norways aircrafts, but since an aircraft can't be used if it's registrered in a non-NATO country, some of the Swedish-registred aircrafts have been re-registred in Denmark and Norway.
Another issue could also be the taxes, which have changed recently. Apparently aircraft-taxes should be cheaper in Norway now.
From what I have heard from a SAS employee, the taxes are the reason, but also they have different classes based on pollusion, noise and so on. At first it would be best to register the aircrafts in Sweden, but then Norway changed their rules and it became cheaper to have them in Norway. That's why they change the reg.
Just like birdwatching - without having to be so damned quiet!
Actually it is mainly because the leasing companies demand the aircrafts to be on Norwegian registry. This is because in case of an SAS collapse, it is easier to reposess the aircrafts if they are registred in Norway ..
There is something called leasing finance. An airline can order the planes directly from the manufacturer, and finance it through a leasing company. Many airlines do that. For example, Finnair has just sold three A320s to SALE/Bollioune and lease them back.