what could be the cause of this? The output power i get from my calculation after measuring AC current with a clamp meter and multiplying by 220v is not always close to the amount of load plugged
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.
What kind of load? Many devices, motors in particular, draw different amounts of power when starting vs. once they are rumning, and circuits have to be rated for the higher number. Many others cycle between different modes: most household appliances would be examples of that. And if course circuits are rated for maximum permitted draw, so adding up the circuit breakers should result in z larger number than what is actually being used.