Airlines that have a range of aircraft types do tend to switch them round from time to time to optimise things, as the loads and markets change. Just a quick look at the SAS 321 usage shows they have just changed it all again. Until last week they were using the aircraft heavily on the Copenhagen to Stockholm shuttle. Next week, with the summer schedules and reduced business demand, it has gone altogether from that route (flights still there but using smaller aircraft).However they are now onto European routes from Copenhagen, to Nice, Rome, Madrid, Frankfurt, Athens, and yes, back to Heathrow on the evening SK 1507 ! Then in August they're switching it to a morning London flight, and the Stockholm flights restart for it.
Someone will have some logic somewhere !