I checked the voltages on two of them, and the hot Roku shows 23.88VDC. The cool Roku shows 23.94VDC. Swapping adapters results in cool getting hot. Clearly it isn't a voltage problem.
I have the hot Roku sitting on top of my Moon I-3 amplifier, and the others are sitting alone on shelves. It looks like the aluminum chassis of the hot Roku is absorbing heat from the amplifier.
So I guess it is just the efficiency of aluminum. The Moon generates heat, which passes upwards through the air, and the Roku absorbs it.