As, electrically "Watts equals Volts times Amps", I would guess that one of these numbers is the power output (from the battery) while the other is the power input (amount of power used to charge the battery); could be that the difference (600 VA / watts) Is dedicated to keeping the battery charged. This particular model has a display that can be set to show how long the battery will last - based on the "load" currently being drawn by what is plugged into the unit; very useful for estimating current power loads of "energy saving devices" - that usually do NOT draw the full amount of their rated loads.