measuring power drawn from an inverter
Welcome to Power Factor. Amps times volts only equals watts for loads that have a power factor of 1.0, such as a pure resistive heater or incandescent light.

Typical loads such as motors LED or florescent lights and most computer power supplies (though some newer ones are well-adjusted and may hit 0.99) have current that is somewhat out of phase with voltage. Some items are actually rated in VA (volt-amperes) rather than Watts, and that commonly reflects a non-unity power factor.

You may also have the simpler situation where you are reading a label with the maximum load and the actual load may be less than the maximum; or the "maximum load" may ignore a starting surge for a motor that may briefly be much higher than the label states for running load.