OK ready for a little old school stuff:
The heat energy in the piston is absorbed by the rings. The rings then transfer this heat into the cylinder walls and eventually into the surrounding air. Since the piston rings are in firm contact with both the piston and cylinder wall, they can 'take' heat energy from the piston and transfer this heat into the cylinder.
Above info from Riken Ring.. One of, if not the, largest ring producer in the world.
When you have less heat transfer with a single ring, the ring gets over-loaded and ring seal fades and with fading you get combustion blow-by(Motor Head above notice much more carbon buildup in the bottom end of single rings)... With blow-by you get power LOSS.. So, if you have blow-by (which you do) and you Lose power (which happens with blow-by) then you will now be over-fueling the cylinder, contaminating the intake charge (due the blow-by) which, in turn, lowers charge purity, which in turn, lowers power... So, now you have made your engine make less power BUT you are still feeding it with the same fuel (via carb jets) for the stock power.. So, now you have created a over-fueling (rich) condition inside the engine.. Rich conditions COOL the piston which is needed because of the ring over-load, and this same rich condition also aids in the power LOSS.. you can not make power with an overly rich condition...
So, it is a cycle that repeats over and over.. ring over-loads, charge density lessens, rich conditions occurs, power output is diminished, piston cools, ring seals normal again, back to stock power (or close to it) rinse, wash, and repeat..
It is VERY hard to seize an engine with a very rich condition... This is a very good reason to use a single ring. Less engine failure due to over-fueling. (some thought? many arguments?)
Remember you are taking compression readings on a NON running cooled engine at very low rpm's.
Detonation is another reason...you cannot pound between the 2 rings since there is only one.One of the MAIN signs of detonation is hammering between the rings (2)
Dyno Horsepower...short pulls to advertise HP..less drag with one ring
Most above info copied from RK Tek 2 stroke....Most is tech info on 2 strokes....food for thought and arguments.....
In the Motocross days we all ran Weisco single ring pistons...and changed them between races.