As exactly said above by pranay, there are some differences between gear and non-gear bikes or scooters on CVT petrol process and its consumption. Few scooters with gear like an old type Bajaj scooter too has problem in mileage but it can carry heavy duty either person or luggage with enough rise and pick up. Whereas, the bikes are with four gears and mechanism is little different compare to scooters.
Thus, four gears or five gears bikes are better in mileage than 2 gears bikes (Samurai, KB100, Yamaha RX100 and on) for average mileage. In addition, if CC increases then there will be issue for mileage as well for four / five speed bikes, like Kawasaki, Pulsar or Honda bikes mileages on higher CC and its less mileage and on.