Why does S-parameter 2-port de-embedding use 4-ports?

In the documentation for S-parameter deembedding https://de.mathworks.com/help/rf/ug/deembedsparams.html#outputarg_s2_params is an example for a 2- and 4-port network. But why does the example for a 2-port network also use 4-ports for the "leftpad" and "rightpad"?