When you use Archive bit option, Second Copy only considers those files that have the archive bit set for the copying purposes. With the Archive bit option if you delete a file on the source then Seocnd Copy would not be able to pick that deleted file in its list of files to be copied, that had the archive bit set. Hence, Second Copy would not be able to match the corresponding file in the destination folder. Therefore with Archive bit options Second Copy does not delete obsolete files in the Exact Copy mode.
If you want Exact copy or Exact compress then you should not use the Archive bit option.