Originally Posted by
danwer930
the current construction closure of a major runway at JFK, some weather at JFK
Both of these factors will affect the amount of holding fuel that will be added during the flight planning process. Delays may, or may not, actually occur but the potential for delay will lead to planning additional holding fuel.
the gas required "to fly at the altitudes required to keep the flight smooth"
Lower altitudes burn more fuel. If turbulence at higher altitudes is significant then additional fuel will be boarded so that the flight can proceed at lower, less turbulent, altitudes. Lower altitudes will also have less wind. On an eastbound flight this would usually mean less tailwind so a longer flight at the lower altitudes requiring even more fuel.
that the plane was full.
You can't fill the tanks with max payload on any airplane. More fuel = fewer passengers or more passengers = less fuel.