Operating Voltage Range Question
It has a nominal power of 2200 W at the nominal voltage of 350 V which, I presume, is DC. This may be related to a standard power supply they manufacture.


  My intuition was that, for a given power supply, it would hold a voltage within a few volts.


No, it would be up to the power-supply to hold the voltage, not the device.


  So what I'm trying to understand, is why a device such as this has such a wide operational voltage range.


Because it can! An electric heating element may have a nominal voltage of 230 V. You can run it at 250 V and it will run a bit hotter (and may die sooner) or you could run it at a lower voltage but it would give out less power (but might last forever).