[crazygringo]

Knowing nothing about it, what causes fuel calculations to be done wrong?

Is it math errors, or uncertainty about exact weight of cargo and passengers, or wind conditions different from predicted, or something else?

[stenius]

Wind is the main one.

You might also run into some weather you didn't have to plan for and that changes the prevailing winds at the altitude you were previously cruising at or causes you to divert to fly around it.