ITunes backups - how's the UID generated?
Given the length of the UID, I suspect the value is a cryptographic hash value. Hashes are often used to avoid collisions and can be recreated if the original input is known.

SHA1 is 40 characters when hex encoded.

We can rule out an OS X uuid and Window's GUID as the lengths are too short. Additionally values returned by uuid on the same hardware include identical segments – unlike a hash.