It doesn't really matter if it's stable if the value used in the national id number is just seeded with the best option at the time it is assigned, though, and

It doesn't really matter if it's stable if the value used in the national id number is just seeded with the best option at the time it is assigned, though, and in that case it can still be convenient to use a value that for most people will be easy to remember. E.g. it's easier for me to remember the 5 digits on top of my birth date than to remember the full 11 digits of my Norwegian id number. That a small proportion of people have leading 6 digits that aren't a perfect match for their birth dates may be a minor nuisance for them but no more *as long as people don't assume the leading digits can be relied on.