It could have been that the factory ran out of the non-rib truck lids since they were getting phased out.
Years ago I went to buy a '76 Dart Lite hood and trunk lid (with the aluminum bracing underneath). Turned out that the trunk was a conventional, all-steel unit. Guy swore it was off of the same car and there weren't any clues to disprove that. Paint matched! They've been known to use left-over, older parts on newer vehicles too.
