Why are Mac files able to have multiple owners?
You are right, vanilla Unix doesn't support this. But there is this thing called ACL (Access Control Lists) which allows for a much more granular definition of access rights than the usual user/group/world rights. From Wikipedia:


  Most of the Unix and Unix-like operating systems (e.g. Linux, BSD, or Solaris) support POSIX.1e ACLs, based on an early POSIX draft that was abandoned. Many of them, for example AIX, FreeBSD, Mac OS X beginning with version 10.4 ("Tiger"), or Solaris with ZFS filesystem, support NFSv4 ACLs, which are part of the NFSv4 standard.


By default every file only has the user/group/world rights



pse@Fourecks:~/Downloads$ ls -le UML_Plakat_8_2012_Web-Download_V1.pdf 
-rw-r--r--@ 1 pse  staff  4914537 Jun 21 09:56 UML_Plakat_8_2012_Web-Download_V1.pdf


If you now add an additional owner



Finder automatically creates a corresponding ACL entry for this:

pse@Fourecks:~/Downloads$ ls -le UML_Plakat_8_2012_Web-Download_V1.pdf 
-rw-r--r--@ 1 pse  staff  4914537 Jun 21 09:56 UML_Plakat_8_2012_Web-Download_V1.pdf
 0: user:jonnyenglish allow read,readattr,readextattr,readsecurity


You can also modify ACL entries directly from Terminal by using chmod (which gives you significantly more details to manipulate than the Get Info dialog in Finder). For details it's probably best to run man chmod in Terminal and read the ACL MANIPULATION OPTIONS section.