Just speculating further … there may be a need to spread users out over time because the servers can only download to so many users simultaneously.
Another complicating factor - and I don’t know whether it applies here - is that if there is more than one physical server for the repo (whether via anycast, CDN or other techniques) then those servers won’t be exactly in synch i.e. an update will arrive at some servers later than others, and you may happen to choose a server that gets the update earlier or later.