Hi everyone,
I have three machines all running EOS.
I update PC#1 before any others and have a Pacman cache web server running on it.
Am I correct in thinking that I need to make sure all machines use the exact same mirrorlist in order to prevent downloading updates not in the cache or am I overthinking it?
Jason
Fortunately, you’re overthinking it! 
A mirror is a mirror and has exactly the same packages all other mirrors have. It’s mainly for easy distribution and having a server near you for faster access.
It can make sense, from time to time, to update the mirror lists, just in case some mirrors have gone away or others become faster. The easy way is to use the buttons in the “Welcome” app. But don’t do that right now, since Arch is under DDoS attack and downloading the mirror list might fail.
“Reflector is a Python script which can retrieve the latest mirror list from the Arch Linux Mirror Status page, filter the most up-to-date mirrors, sort them by speed and overwrite the file /etc/pacman.d/mirrorlist.”
This implies that not all mirrors are the same. I’ve realized though that EOS doesn’t get the list of updates from the mirror (this thinking was what was confusing me) but the main package list so I don’t need to worry about mirrors on the other machines at all really.
Well it is like this packages that are specific for EOS come from the EOS repository that has its own mirrorlist. Other packages that come from arch (including AUR packages) come from the Arch repositories that also have their own mirrorlist. If you look at the welcome app there are two options for updating the mirrorlists.
This is true. Out of Sync Mirrors and Successfully Syncing Mirrors depending on the delay, they may be in very different states.
They’re the same in the sense you can download Arch or EOS packages from any of them and the most up-to-date today could be different later.
Yup. @keescase, @EOS and @vazicebon nicely explained the rest of the story! 
EOS has its own mirrors (for EOS-specifics), and Arch has the “main” mirrors (for all Arch-based things).
Of course one can’t keep hundreds (thousands?) of servers perfectly in sync all the time; development goes on and updates arrive all the time. Also, some servers can go down due to wars or technical problems, that’s what the “most up-to-date” comes from.
So all good, @kokochu!
Yeah like I say I had it in my head it each PC was looking at the mirror and saying “oh look, an update” rather than looking on the package database which I now understand is central and only the packages them selves are mirrored.
Thanks for the info guys.