hey-Hey!!!,
I had to smile at your screen wattage testing. I once ran 85W through the g2 of a 4E27 I was testing when I forgot to put B+ on the anode. It survived OK...
As to the differences, which data are the 100V/100 mA data comming from? I assume your measurements? I can't offer anything concrete, but a good sounding power pentode with a 60W plate is not going to get a very close look from me under most circumstances, eh?
cheers,
Douglas