Bookworm,
I do think a decrease in pumping losses could well be at least part of the answer. I understand that some engines used for pumping water use the weight of the "falling" water to cause a low pressure extraction effect in the exhaust, which is like the altitude effect we are looking at here.
Similarly, a well tuned exhaust pipe can have a very significant effect on power and / or economy on any piston engine. This occurs because a low pressure pulse in the exhaust port can be timed to occur in conjunction with the exhaust valve opening.
In a way, it's a little like a "negative supercharging effect", at the opposite end of the engine.
Interesting topic!
P.S. Try this link for some bedtime reading:
http://naca.larc.nasa.gov/reports/19...-report-45.pdf