I have no personal experience in that particular part of Norway and cannot speak to whether or not the naming customs were "customary" there and at that time.
Have you consulted the bygdeboks that might exist for those areas? You may not know about all of the children that were born and therefore be missing names that were actually given. Or that couple might just not have considered the naming patterns to be important to them. There was no rule - it was just a custom, the usual pattern.http://homepages.rootsweb.ancestry.com/~norway/bygdebok.html