There are four numbers to consider for a successful task:  succeeded, attempted, skipped, and total.  If there were failures with individual emails, then succeeded will be less than attempted.  

"Unable to find any recipients to be emailed"  (total = 0)
"Message failed to be emailed for any of {attempted} recipients"  (succeeded = 0)
"Message successfully emailed for {attempted} recipients" (succeeded = attempted)
"Message emailed for {succeeded} of {attempted} recipients"  (succeeded < attempted)

"skipping" is appended if skipping > 0
" out of" is appended if attempted < total (i.e. due to skipping)

So I guess that one will only see the "out of" if one also sees the "skipping".  (I just didn't code it that way.)  That is, I expect that attempted + skipped = total at the end.  (It's not the case during progress, obviously.)