Working Set- Changed today
will indeed only grab files change 00:01 or later of the current calendar day. Even if the job starts before, but continues after midnight, I
think that the files updated before midnight will be skipped, too, but I'd have to play with it to be sure
It appears that the first job that is running is resetting the backup date so that the next job won't back that file up causing the byte count to differ.
You can can change the backup method on all but the last to COPY and they will not reset the Archive Bit, so that the following jobs can look at it.