hi BMM,
i guess in this case there are 2 variables i.e. weight and altitude. actually, the 3rd variable would be the holding speed but i guess in this case it assumes the best holding speed from FMC.
for a given weight, you will noticed the best FF will be achieved at a certain altitude i.e. optimum altitude. if you move away from that alt, either up or down, the FF increases.
so, in your example for 240T the opt holding alt is 15,000' whilst for 220T the opt holding alt is 20,000'. obviously, the FF for 240T will be higher than 220T i.e. 3350 Vs 3050.
hope this helps...
SR