'rsync -a' not preserving ownership

[cygwin@kosowsky.org]

I am running cygwin as Administrator and tried this under both Win7
and Win10.

# touch test
# chown Administrators.SYSTEM test
# rsync -a test test2
# cp -a test test3
# ls -al test*
-rw-r--r-- 1 Administrators SYSTEM 0 Jan  3 11:00 test
-rw-r--r-- 1 myname         None   0 Jan  3 11:00 test2
-rw-r--r-- 1 Administrators SYSTEM 0 Jan  3 11:00 test3

Note that perms are copied properly (which I validated by trying with
other perm combos)

So, why is 'cp -a' able to preserve ownership while 'rsync -a' fails
and sets ownership to the login name?