It is actually incorrect that the pressure inside the cabin is lower than that outside, it is lower than the free stream static pressure, but not that immediately outside the cabin.Another way to think about it is that the air moving round the airframe will have different pressures in different places - the reason you have a lower pressure inside the aircraft is that the air moving round the cabin itself accelerates causing a lower pressure just outside the cabin, basically sucking the air out and so reducing the pressure below the free stream static pressure, so trying to open the DV window is doing it against that "suck". This is also why, as mentioned before, the designer has to be careful of the sitting of the static vents, otherwise it will be measuring the pressure of the air affected by the fuselage and not the free stream static.