Why is this device power usage so much power and with such variation?
If your watt meter is crappy, it won't take into account power factor (but then I wouldn't really call it a wattmeter anymore since the whole point of a wattmeter is to see through the power factor). I kinda doubt your drugstore humidifer is drawing 640W. 

You can't calibrate it. Voltage and current have to be in phase for real work to be done. When they are out of phase you get reactive power which is just real power moving back and forth not doing any work. To measure this it has to measure instantaneous voltage and current instant-to-instant. If your meter just measures voltage over time, and current over time, and then uses those two numbers to calculate power then it ignores the phase information.

It is like a warehouse that has imports and exports. If your imports and exports are staggered then your warehouse isn't ever really overfilled. But when not staggered you get a multiple consecutive import shipments accumulating in your warehouse before an export shipment can empty it out again. But you cannot see this effect if you just examined the average number of exports vs imports over a long period of time such as year or month. The phase information between imports and exports is not accounted.