I agree with the earlier comment about correct pax and bag weights. The airline I work for did a survey over a period of time and the weights were found to be consistently under. So they came up with a new set of weights so that the ZFM was realistic. Crews began to see that an accurate ZFM resulted in landing with excess fuel and began reducing fuel loads as the estimated weights were now actually fairly accurate. Then the airline merged and we are back to square 1.
Pilots take extra fuel because in many cases the estimated load weight is wrong, and lets face it, when you get the actual loadsheet (around 2 minutes before pushback!!!) you are hardly going to say "hang on, get the fueller back, we are heavier than planned and I want an extra 200kgs" now are you?
So - note to airlines - provide pilots with accurate estimate load figures and they can calculate an accurate estimated fuel load!!!