Educated guess: If you are using a "certified" calibration tool for VNA which may be pre-defined(some VNA's ask for which calibration you use), then it may subtract the effects of calibration open/short from measurements.
Think of ports as your eyes and in the circuit; and when u calibrate a VNA(it may inclue cables or not), end of the part which u performed calibration becomes your eyes. Then you add some adaptors or jack-to-sma interconnects to the DUT. When you are adding the adaptors, each adaptor presents an "electrical length" and "insertion loss" for their imperfections. Port extension meaning is that u write the "electrical length" and "loss" of those parts into your measurements to account for *phase difference (mostly) in your setup. When thinking about magnitude, port extension does not make sense(most of time); but If you are insterested in "phase" then it's useful.