As I explained earlier, there is a parallel path for the neutral current to return to the service head. One that results in a small proportion of the neutral current from the socket circuits, but not the live current, flowing through the RH RCD, thus unbalancing it.
The proportion of diverted current is the ratio of the resistance of the neutral tail compared to the resistance of the RH circuit neutral, fault resistance, RH circuit cpc, and main cpc, which can be a very small proportion, BUT 20mA is a very small current, when compared to the many tens of amps that can flow as an inrush current on the socket circuits.