Due to panelboard OCPD requirements, or just that there is a lot of 120V loads that due to load transfer/movement may result in a lot of imbalance for a short period of time, detection of a 120V overcurrent is made more accurate by secondary protection. Remember the primary to secondary is not bolted, so a significant single phase load change does not directly effect the primary current per phase in exactly the same ratios (ie primary phase A to secondary phase A).
