Many reasons... Did you have the throttles to the fire wall?  If your throttles are full open then you will use more fuel than if you were to use an economic airspeed.  Did you have a headwind?  Head winds slow you down and you increase your throttle to make up time, increasing your fuel consumption.  Did you create the flight plan then choose what airplane you want to fly.  The navlog will give you an estimated fuel consumption for the aircraft selected.

Can anyone think of anything else?  In my opinion the navlog give a pretty good estimate on how much fuel is needed for a flight.  It can be pretty close, but sometimes I have found it to be way off for the aircraft I was flying.  So I usually take what the navlog says and then increase it so that I have plenty of fuel for the flight and then some for the go around if needed.

when navlog calculates the fuel quantity, it doesn't consider taxiing, so add a little extra for taxiing.

about the headwind...is this effect simulated in the fuelburn?

and as said before, don't put your throttles to the max, as soon as you are of the ground, put them to 90% N1 and at cruise altitude at lets say 70%N1