You'll see this characteristic in most fuel consumption charts that you look at. If you also look at engine power charts, you'll typically see power increasing with altitude below the FTH. An offhand example of this would be the P-51D. At 61" and 3000 RPM, it has 1490 HP at SL. At the same MP and RPM that increases by 100 HP at 8 or 9K.
As Gripen points out, the supercharger is doing wasted work below FTH but there's more going on than that. In fact, it's actually taking more power to drive the supercharger at FTH than it is at SL even though it's more efficient at FTH than it is at SL.
We tend to think of constant manifold pressure and RPM as constant power or a constant amount of fuel/air charge to the cyclinders, but manifold pressure and RPM are only two parts of the equation. There are a number of other factors involved but the other two major ones are the exhaust backpressure and the charge temperature, both of which are affected by altitude changes.
The density of the fuel-air charge is proportional to intake pressure / charge temperature. So as altitude increases and temperature decreases, your charge weight will go up for a constant intake pressure.
The other major factor is that as altitude increases and ambient pressure decreases, the lower the pressure of the residual exhaust gas in the cyclinder which means more charge can enter the cylinder during the intake stroke.
A further factor could be whether the automatic mixture control on any of these planes adjusted to this but I don't know if that was the case or not.
