Some key facts and assumptions
My engine is from Tornando Alley and operated with the recommendation of WOT at all climb and cruise operations.
15.5 gph is about 2 gph less than their 'Go Fast - 85% power mode' so it is well on the LOP side (maybe 150 degrees).
Because (like all of our engines) it has fixed timing and the flame front will be much slower this lean, the peak power pulse will happen much later than optimal. Therefore it is cool, but also probably less effective.
At 2200 RPM the amount of air being pumped per minute is less at the same fuel flow so I am closer to stoichiometric (closer to peak EGT) and the slow RPM means the peak pressure will be more in line with the optimum crank angle. Hence more efficient conversion of fuel to speed.
At least this is the logic I use to explain the efficiency improvement