NOTE: This is NOT a domain. This is my own simple home network with 2 Ubuntu machines with the same login/password credentials.
My guess?? I suppose that a login name has more to it that I can't see (don't understand) and that ownership of a file is not solely based on the login name. Perhaps there's hash or metadata or something that is unique - even with the same user names. Or maybe Samba strips unix credentials.
No hash or metadata. The user is the owner if he created the file. The user is a you want to define it (by prior default or configuration) such as in the Samba configuration file.
I guess the easiest place to start is: Is this a Samba thing or a linux security thing (if that even makes sense).