The Cidr Report

One possible explaination of the bouncing is that the number is snapshots
of the total routes of a router. If this router happened to loose bgp
session with a large ISP at the time the snapshot is taken, the total
number of routes of the box could be hundreds or thousands routes less
(depends on which ISP it lost peer session with). About 10 or so ISPs
peering at mae-e announcing more than a thousand routes.

If the reporting data is based on one snapshot a day then the chances of
encounting the described above is more than if the data is based on
several snapshots a day.