It probably has only two receiver channels internally instead of three or four. They may have a single directional coupler (more likely a resistive bridge) on the main port to support S11 measurements, and a conventional non-directional ALC sampler on the TG port. S21 measurements would be scalar only, implemented by treating the tracking generator output as a transmit-only "port 2."
Not necessarily a bad way to go, at the right price. But it limits your calibration options. It had better be cheap... very cheap.