The solution to this issue was upgrading the client to 8.2.
There must have been an issue in the Legato Networker version 7.6 on Linux: consistently, unchanged files went into the backup. We had this issue, event with Networker 7.6 on client and server.
Until now, I have not been able to locate the reason why so many unchanged files have been saved: I cannot find a consistent pattern. The archive bit appears to have set me on the wrong track.