Another variable is winds aloft. Winds aloft are always from west to east, and the schedule is based on typical winds.

If either flying into headwinds less than forecast, or tailwinds stronger than forecast, you have a choice.
1 - Decrease power to maintain planned groundspeed (much more fuel efficient), or
2 Maintain the planned airspeed (higher groundspeed for the planned fuel burn).

It's an easy way to make up time for no fuel penalty.

Works opposite the other way. If higher headwinds or lower tailwinds than expected, it's very hard to keep on schedule no matter how hard you push the throttles.