That is a combination, with US gallons (3.78 litre) which are smaller than imperial gallons. Note the fine print:
"All mileage figures are from the [US Government] Environmental Protection Agency's recent test findings"
So all the cars in the list would have been tested the same way. Back in '73 they reported a combined figure instead of city/highway.
* MOTOR (NZ) said "Average on test, 28.9mpg."
* Road & Track test reported 28 mpg
* Consumer Reports said "23-41"
50mpg was Datsun B210 "MPG" model (A14 round port engine with 5-speed).
Fuel_Economy