Most likely because what is going on is not related to the official repositories
Unmaintained, deprecated and otherwise obsolete packages may be moved to AUR.
When this happens packages will be left on the system as unknown packages.
It is the local system administrators task to keep the system in a tidy state.
But it is common knowledge that this is often left to a better time and because that better time never arrives, such maintenance to go forgotten.
That fact is one of the reason why the update announcement contains a shortlist of preparation steps - take the time every once in a while to go over the guide and clear your system of obsolete and abandoned packages.