They get the people with the most experience and knowledge in the area and it's likely that those people will be older, pre-written obits are for the big names and when those people, like Jimmy Carter, live an extraordinarily long life, it shouldn't be too surprising if the obit writer goes before the subject.
In Nolan's case it seems a bit weird to have an obituary on ice for at least 6 years, but the guardian seems to have a massive backlog of pre-written obits, covering anyone who has even been within an inch of fame.