When uninstalling MC, it appears that MC automatically creates a library backup.
This is not correct. I wasn't 100% sure so I just tested. Media Center's Uninstaller does not make an automatic Library Backup before uninstalling.
It does, however, make the automatic Library Backups it makes at
launch. I suspect even at
first launch (after all, it is "late" because it has never made one before), so what is happening to you is that:
1. You're uninstalling for whatever reason.
2. Reinstalling.
3. Launching MC.
4. As it launches, before you restore it or anything, it makes a Library Backup just as part of the normal automatic Library Backup system.
5. You're picking
this Library Backup, which would be pretty much a clean slate (unless you have an older install of MC on that system, in which case it would be the automatic upgrade of MC20's settings, or whatever).
In any case, it does not make an automatic Library Backup when you uninstall. So, I'm not sure where the "different" one is coming from, but I don't think it is coming from where you think it is. The backups MC makes are all the same, and should capture the current settings, and all of them, including toolbar buttons.
If you can find a repeatable instance where this isn't the case,
exact step-by-steps would be good.