OK. These numbers are based on an extract from a power setting table for a
Lycoming 180HP :-

For 55% rated power with a FF of 7.4G/h at SL with an RPM of 2100, the MP
is 20.9.

For 55% rated power with a FF of 7.4G/h at 12,000 ft PA, again with an RPM
of 2100, the MP is 18.2.

I understand (and please correct me if I'm wrong) that fuel flow has a
linear relationship to power, i.e. a certain fuel flow will produce a
certain amount of power. Therefore, the FF figure of 7.4G/h producing 55% at
both alts is consistent with that understanding. However, what I don't
understand immediately is why at altitude the MP required is only 18.2
rather than 20.9.

Anyone have a simple explanation?

Thanks in advance. I'm working on the "no stupid questions" maxim here....
although some of this stuff makes me feel like a moron from time to time.