There are a bunch of background jobs that are spread out to manage the database load... but the site activity does not stop.
Sometimes there can be activity between two jobs that calculate and cache similar information and they might be each correct at the instant they run but different even when run 15min apart.
If the difference is always exactly the same gap between those ranks, that's odd, but I am not surprised if there is a small variation due to different caching on different places on the site to help with performance.