The quantity and the intensity of deep Slow Wave Sleep (SWS) varies from one sleep cycle to another and even from one night to another.
We often talk about SWS pressure, because the more you are sleep deprived, the more easily, rapidly and intensely you will fall in deep sleep. In such cases, RMES quantity decreases to extend SWS.
Moreover, SWS varies a lot during your life. Although newborns spend tremendous amount of time in deep sleep, it is less present as we are getting older. It can even disappear totally for some retirees.