a lot of things can be taken into account, weather for example because ur car uses a lot of petrol when starting up while cold. If its warmer then it wont take as long to warm up.
Other things like which fuel u actually use. You will in most cases get better milage out of 95 ron than standard 92 ron unleaded.
For me, fuel makes a big difference. i usually get 400-450 kays out of my DC5R (yes different car but its still 50lt) but if i use certain petrol (caltex 98) i can get it to 500-550 without too much troubles (and that includes vtec and some heavy throttle)
a guy at work owns an 07 civic vti and he gets betwen 500-600 kays from his car and i tell u now, he DOES drive like a grandma but he also takes a lot of care and the car is always garaged (weather, engine doesnt get quite as cold).
Check your tyre pressure if you're worried so much and maybe get a wheel alignment as well and dont forget, an intake mod will most of the time increase your fuel consumption unless its tuned to use it efficiently!