Wattage ?
they could be calculating total wattage handling, heat and internal wattage dissipation
out amps a 24v is less than at 12v for the same watts, but internal load on the components involved could be more, with the higher input voltage into those solid state parts
in other words, its NOT linier its not the same amps for each voltage
, like just about every other mfg, not 800, 1600, 3200
i'm only taking educated guess's, but that what it seems like
as voltage input/output goes up they have derated the max amps
to compensate for the internal stress produced by the higer voltage
