Here is how to demo the problem:
Grab Terminal on OSX, and run
cd ~ && mkdir abc && ls -l ~/
Here is the output about folder "abc":
drwxr-xr-x 2 justin staff 68 Jan 4 20:20 abc
The group name can be seen as "staff".
rsync a file or folder from remote server to the folder "abc":
rsync -aP -v -z -h username@server:/mnt/foo/ ~/abc/
check the group name of the folder "abc" again:
$ ls -l ~/ drwxr-xr-x 2 justin _lpoperator 68B Jan 4 19:44 abc
and all the files in "abc", the group names are _lpoperator. Try to create a file in "abc" and check its group name,
$ touch def $ ls -l ~/abc -rw-r--r-- 1 justin _lpoperator 0 Jan 4 20:28 def
I know when i use -a option for rsync, it will try to keep the permission for that remote file, but why _lpoperator was created? it's really weird, any idea?