Thanks to simple statistics (and the biological immortality old age endows), any given world's oldest person has an expected incumbency of about two years.
It may have something to do with their newfound status, all the excitement around being the oldest person in the world can't be too good for the health of the oldest person in the world.
To increase the frequency a bit further there is the 'oldest person', the 'oldest female' and the 'oldest male' (this doubles the number of reports).
http://en.wikipedia.org/wiki/Oldest_people
This year alone there have been three such reports.
It's also a fact that someone has to be the oldest, who it is is not that important.
Just like the tallest tree in the forest is not very significant, it's just another way to look at the bell curve, there will be a few short trees and a few of them will be tall, most of them will be in the middle.