Wikipedia defines Curb Weight as:

Curb (kerb) weight is the total weight of a vehicle with standard equipment, all necessary operating consumables (such as motor oil and coolant), a full tank of fuel, and not loaded with either passengers or cargo.

I would bet that the difference in a half tank of fuel is quite a few pounds. How much does one gallon of gas weigh? A gallon of water is 8 lbs. isn't it?