I have a Mac Mini Server with two internal SSDs. Until today, I had only ever installed an OS, Mt. Lion, on one of those drives and the other had remained largely empty, except for when I needed to temporarily store files on it.
Leaving the Mt. Lion boot drive untouched, I did a full erase of the empty SSD and performed upon it a clean install of Yosemite from a bootable flash drive.
After completing that and starting up with the new Yosemite drive, I declined to transfer any files, applications or settings, did not sign into iCloud and did not even set up DNS servers in Network Preferences. I created a new admin user with a new name and password and used FileVault to encrypt the drive with an offline recovery key.
While poking through Keyboard preferences, I have discovered numerous Services that I installed on the Mt. Lion boot drive, the majority of which correspond to applications which are not even installed on this Yosemite drive. How did they get transferred from my other boot drive when I've intentionally transferred nothing and have not logged into any previously existing accounts‽
EDIT: In response to the comments, which have been added: (1) None of the Services or Scripts folders on the Yosemite drive contain anything but the defaults; (3) the user accounts have completely different names and the Yosemite install is not tied to iCloud or any other service that would tie a user account across multiple platforms; (2) and, yes, it must be reading them off the Mt. Lion boot drive but how and why? Shouldn't the very fact that they're completely different drives with different OSes installed upon them and not even simultaneously bootable and with no users accounts in common or weird symlinks set up make that impossible??? And why is Yosemite looking outside its installation for Services to begin with?