Because most machinery present an inductive load and so has a less-than-unity power factor. The current peak lags the voltage peak in the two waveforms. However, as the current is drawn at a lower voltage (away from the peak), the load current is higher to get the same overall power to the machine. eg. if it took 20% more current to make up for the voltage phase offset, a 1000W load would be taking the same current as a 1200W load at unity power factor. That's the VA rating, it's equivalent to watts at unity power factor.
