Not possible. Even with a faulty carb, you will be able to get more than 30km per liter. So how can you get 18?
Either you are measuring it wrong, or Petrol Pump is messing with you by not giving full fuel.
Also, you should know that brand new bike DRINKS the initial 2-3L because fuel has to reach deep down in the engine. So that first liter you put in it disappearing so quickly makes sense.
You are measuring it wrong. Fill the fuel tank, wait for reserve. When it goes in Reserve mode, fill it up again and measure the amount put in. Divide that amount (say 6 litres) with the kms driven (say 300kms)
That'd be 300km ? 6L = almost 50 km per liter.