Title says it all but just to clarify. I did a very unscientific test in a quick battle. Told my 2 units of archers to aim at some Feudal knights that were intermingling with some mounted sergeants. in 5 volleys a good ten knights fell but no sergeants. I have camera lock on (a feature sorely missing in the newer titles IMO) so i couldn't see exactly where the arrows were landing, but probability was on my side.
So can arrows kill things they aren't aimed at? I've always assumed so but never actually known.