Irlandes,
I think the problem is that you are expecting the hover performance of the aircraft to be driven only by the density altitude. For any helicopter, the hover performance is based upon the altitude, temperature and engine power available, as well as the ancilliary limits, such as tail rotor effectiveness.
The gain in hover IGE weight with reduction in altitude is a product of the both rotor efficiency, (which is almost entirely driven by density altitude), and the engine power available, which gains power as altitude is reduced, and also as temperature is reduced.
I believe the hover performance is more strongly influenced by engine power than by rotor efficiency, and you have deduced that from the charts.
As an experiment, look at several constant density altitude conditions, where the temperature is increased considerably, and see if the weight to hover is a constant. I believe the lower temperature conditions should have higher HIGE performance.