Actually, I have a theory about that based on my own observations that HEL routes of SAS are mostly half empty in the business hours for business travellers, and extremely full on the last flight of the day. For example, If I fly HEL-ARN/CPH at 10 it’s highly likely that on an average neo aircraft 1/3 is empty, sometimes more, sometimes less, but rarely fully booked. But last flights are always fully booked or close to that.

Generally, SAS serves HEL very poorly, especially out/to ARN where on a good day there are usually 4 flights which one could get cancelled and in summer/weekends it could be only one/two flights per day! If you look at fares they are extremely cheap starting at 45€ while AY charge much higher.