On 22 July 2011, he killed eight people by detonating a van bomb amid Regjeringskvartalet in Oslo, then shot dead 69 participants of a Workers' Youth League (AUF) summer camp on the island of Utøya. In July 2012, he was convicted of mass murder, causing a fatal explosion, and terrorism.
On 24 August 2012, Breivik was adjudged sane and sentenced to containment—a special form of a prison sentence that can be extended indefinitely; with an approximate period of 21 years and a minimum time of 10 years, the maximum penalty in Norway. Breivik did not appeal and on 8 September media announced that the verdict was final.
Anders Behring Breivik, Wikipedia
I don't get it: he killed 77 people, but these murders were combined into a single crime "mass murder". It's like he gets a discount. Sure, it's the maximum penalty, but shouldn't he have received 77 penalties?
Question: After killing 77 people, why was Anders Behring Breivik not convicted of 77 murders?