Crab- I agree that DA is DA as far as the rotor is concerned (compressibility effects ignored) . But as the engine sees things, though, DA is not just DA.
On the bit of the graph that Irlandes was looking at the engine's throttle is fully open. The power output of the piston engine is not solely dependent on density altitude, but all kinds of thermodynamic boffin-equations. So for example- although 9000 feet and +10 degrees is the same DA as 10,000 feet at 0 (more or less) the engine is putting out less power in the latter case so the rotor can do less work.
You'll see the same thing with any helicopter once the engine starts to be the limit- in turbines it'll eventually be the TOT or N1 that limits you hot and high. If the engine can't put out the torque the rotor can't do the same work for the same density.
There's no mystery to it (despite my earlier, misguided attempts to muddy the waters) and we can't conclude crappiness at all!!