Endpoints Repository is a list of every MAC address that has authenticated to ClearPass or has been imported from some external context server, like AirWatch. The Guest User Repository is the location where Guest User accounts are created and live. To add to this, the Guest Device Repository is a seperate MAC address database, which is devices that have registered on the guest network or through MACTrac functionality, I think OnBoarded devices will also show up in this list.
I would confirm the MAC address in the Endpoints Database, when you find the MAC, click on it and see if there is a conflict shown in the details. It's a possible case of MAC spoofing, but sounds unlikely. I think with Android you can manually change your MAC address, so it's possible the client has a custom MAC.