When you won in first fight- you are back as "available target" and another person can try to plunder you. If you win 3 times like this- then you are no longer on list of targets and can run your convoy safe.

But when one person suceed at defeating you- then nobody else can fight you over this convoy anymore.

And of course if you deliver convoy to the end without any one defeating you on the run- you won't lose any coupons.