Hydrostatic pressure build up at interfaces
 

Hello there,
I have a model with 3 layers. I have specified a pressure of 0Pa at the surface of the top layer and 80Pa at the bottom of the lowest layer. I have set the operating density to the lowest domain density and enabled gravity, this works for a single domain without interfaces. However when I include interfaces, there seems to be a build up of pressure around the interfaces, could anybody tell me why this happens?