Selecting "preserve atime" means that backup operations of that domain will attempt to reset the UNIX/LINUX access time of each file so that the backup does not appear to have accessed the files. However, because of a restriction in UNIX/LINUX, resetting the access time will change the modification time of the file to be the current time. If preserve atime is not selected, the access time will become the time of the backup (as is normal for a "read"-type operation on a file), but the modification time will be untouched.

Level-1 and Level-2 backups refer to which time (create time or modification time) will be used to decide whether to include a file in a Differential or Incremental backup. Level-1 selects the "create time", which is always unaffected by the backup. Level-2 selects the modification time. If you select both Level-2 and preserve atime, on every backup after your first backup, every file will be backed up because preserve atime changes the modification time of every file to the time of the backup!

Either switch the backup type to Level-1, or disable preserve atime.