I have long pondered this , too , and have routed the cable both ways. Both seem to have logical advantages and neither impressed me as the definitive "correct" way for a bare cable.
I like the idea that the lower tab might be the one intended for a housing, but I'm not sure it would afford a stop at the end, which it has to have to work against the stop on the FD.
EDIT: I'm revising my position, and base it on another examination of the campy unit as well as the Zeus version (which some might say faithfully copies the Campy). The small lower lip routes the cable to the same position as the RH tab routes the cable for the RD, so I think this indicates intentionality. The Zeus version is made the same way, even has the little tiny tab on the edge between the lower and upper lip that the cable has to pass over...BUT, the upper lip doesn't have the guide finished at the "leading edge" such that a cable routed just to that lip would stay in place: it would ride up even higher than in the Campy guide.
I think this was intentional design by Zeus to discourage making that "mistake".
Check out the pic from Velobase...it's even more apparent when you have one in hand to compare to the Campy.