question on permissions

[Sri Ramkrishna]

I have a question regarding how rsync changes ownership when syncing two
areas.  Currently, I have this situation:

I have two areas over a WAN, we are trying to mirror from one site to
another.  One site is not controlled by us and has different unix
groups.  

When we copy one to the other, we are running rsync on an account that
exists at both places (different UIDs though)  In one area we have no
problems the groups/owners exist at both places.  However in another
area, the owner exists but the group name does not.

When we run rsync between the two we get a lot of chown errors.  Now
this makes sense as some OSs (we are using HPUX 11) do not allow a
person to change the ownership of a file.  However, we have another area
where both the group and the ownership exist on both ends but we have no
chown errors.  Can anybody explain this behavior?  In the end, the area
still gets owned by the uid and gid of the rsync running on our end.

Thanks,
sri