Well i called no one a liar, i simply said you were wrong because. well... you are.
Here are pics of a 400w magnetic ballast, it is drawing 480W due to its inefficient design (being magnetic) it draws more than the rated 400w.
This last one is with my fans plugged in and my hydro unit going. 600W
Are you sure you aren't using a US scale electricity meter or perhaps your meter is switched to 120v? In the states we run 120v opposed to your 240v. If you are using a meter meant for 120v it may show half as many watts because it is passing less amps through it and converting it incorrectly.
Perhaps you are measuring before the light fires? when i hot start my lamp it shows about 170w until the lamp re-ignites, at which point it goes back up to 480 W
Don't need to take it personal man