It is simple since for example you have 2 cells with pressure 10 and 100 bar if you will average them you will get 55 bar
but what if cell with 100 bar have 0.5 pore volume and cell with 10 bar have 1 pore volume then your average pressure would be
10*1 + 0.5*100= 60
0.5+1=1.5
60/1.5=40
Think about this
Why you need to wheit over pore volume , because it contains what you are about to develop