This is this way. Drain to Bulk potential another second order effect that creates extra careers and adds up in the conduction.
When you add body effect, Means when your body and source are at different potential, your conduction gets affected called body effect.
For NMOS case, if source is at higher potential than bulk there will be large voltage difference from drain to bulk and hence more carrier. You get extra current that flow from drain to Bulk. In another term bulk potential difference adds up extra resistance, rdb = 1/gmb in parallel to 1/gm . total impedance at source of diode will be (gm+gmb)^-1.