All things being equal, you can blame the inefficiency of the automatic transmission (CVT), though manufacturers keep saying it is only about 10% difference, actually the loss is more like 30% - compare the motorbike with manual gear (honda CB Twister 100cc bike with same weight as Dio) with Dio(autogear with 100cc and same engine) and see the difference [Obviously, under similar driving conditions].
See SIAM mileage chart for DIO vs Twister. 56.6/85.7 = 33% difference
Same for Suzuki Access (Automatic) vs Zeus 60.4 / 78.9 = 24% diff
Obviously you cant trust SIAM's actual numbers, but it does show that automatic is very inefficient and that the manufacturers have done nothing to improve the efficiency of the CVT!
Though computer controlled (robotized manual) transmissions give as good or better mileage than manual transmissions, this will not come to India.