the intensity in a stack after interferogram calculation is no longer the single dates’ intensity. When you open their properties you can see something like this in the pixel expression:
i_ifg_IW3_VV_03Feb2017_23Mar2017 * i_ifg_IW3_VV_03Feb2017_23Mar2017 + q_ifg_IW3_VV_03Feb2017_23Mar2017 * q_ifg_IW3_VV_03Feb2017_23Mar2017
A intensity measure of both dates is somehow generated. Although I’m not sure about the theory or aim behind it.