Did they narrow the spacing of the two top tubes. If they weren't lugged they could start closer together on the head tube.They would remain closer together the entire "trip" down to the dropouts.
I think this is exactly what happened. The connections between the two top tubes are lugged on my '84 Nishiki, but not lugged on the '86, and the tubes are indeed closer together on the '86. Why somebody thought this was a good idea, even though it necessitated that hideous bulge/bend to avoid rubbing the chain, is beyond me.
I wonder if this has anything to do with the fact that the mixte sort of died out around that point?