Cheaper hubs, mostly. The Varsity might have been the bottom of the Schwinn ten-speeds but it was built like a tank. It could take a phenomenal amount of abuse and keep rolling. It's not unusual to find 50-year-old Varsities still rolling on their original wheels and drivetrain.

A Traveler in the '70's was similar but "Schwinn-Approved", that is, made in Japan to Schwinn's specifications. It would be lighter than the Varsity due to its lugged frame. Still a basic, heavy, ten-speed.

Interestingly, the 1976 catalog had them on adjoining pages:
Schwinn catalogs, 1971 - 1980 (242 of 579) (waterfordbikes.com)