Hmmm. Thought one of the real engineering types would venture in here. In the meantime, just the musings of another pilot who tries to retain some of his 6th form physics, so take with a grain of salt (KISS as always)
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As Tinstaafl and others points out, the jets' guts are optimized for operation at high altitude where they spend most of their time, therefore they will have somewhat poorer sfc lower down, but where the efficiency of the
aircraft is concerned, that isn't the dramatic difference.
Jets do their work by throwing masses of air out the pipe at a velocity, at a rate, which is pretty much force=massxacceleration. The forward velocity (TAS) of the A/C is largely secondary. Consider that the thrust (=drag) to maintain 250-300 CAS (about what jet airliners cruise at) will be similar at MSL as it is at FL310 and above. Thus, the fuel burn at 250-300 (CAS=TAS) at MSL is similar to that at about 440-480 TAS at the FL's (250-300 IAS!), actually a little greater due to the inefficiencies to which Mr. T. et al referred.
I know I don't wish to read several long tomes to answer every little curiosity I have, but if you do want to read up a bit, in addition to the other books referred to, you might look at
MECHANICS of FLIGHT by A.C. Kermode. If its in your nearest library, the index will direct you to the dozen or so pages that will explain it all quite nicely.
Cheers, Atlas