Fuel consumption is related to HP output. If the fuel consumption is going up, so is the HP--often max HP is at top RPM. There can be all sorts of things going on--more injector pressure, different timing of injection and valves. I suspect that you are having more effecient use of the engine at mid to high RPM, then at max RPM the fuel consumption goes up.
Unfortunately outboard manufactuers don't often post RPM/Fuel/Torque and fuel consumption curves. Just prop test curves can be miss leading because of the dynamics, of the boat hull, prop interaction and drag factors as the speed increases. Boat speed and HP are not linlear functions.
