Think of it in these terms: Electric current will seek all available paths back to its source. Current that leaves a transformer, flows through a panelboard and on to a load, will return to the panelboard and to the transformer, before starting its circular journey again. This current will not flow back to the main panel, and does not flow through the main meter. It is a separate system. It is created, or derived, by the transformer, and the absence of a direct connection between primary and secondary windings is what maintains the separation.