There are two commonly used methods to calculate the apparent power (VA).
The traditional method is the vectorial calculation. Here we are adding the active power (W) from each phase and the reactive power (var) from each phase. As the reactive power vector has a 90° angle vers. the active power we can calculate the hypotenuse (VA). Old meter installations are using this method.
With electronic meters with microprocessor we are able to calculate the VA for each phase individually and then add the VA values. This method is called arithmetic calculation. It is more accurate.
CLOU meters are using the arithmetic calculation. Some meter types can be set to vectorial calculation mode.