So, to recap
FMGS' ALTN predictions are based on:
- CI 0
- cruise at FL220 if dist. to altn less than 200nm; FL310 if more
- no wind unless an altn wind was inserted in the DES WIND page
- direct routing unless modified by pilot.
All of the above have an impact on the displayed altn fuel required. The more parameters you modify iaw. the flight plan, the more the fuel prediction will match you FP's. If there still is a discrepancy after that, take the safe option ie. the highest fuel figure and you can't go wrong.