At low temperatures, z falls below 1 and the reason for that is because the intermolecular interactions cause the pressure exerted to be lesser than expected. PVm/RT=z and since P is less than expected z drops below 1. However, as the pressure increases z increases to be above 1 because as P increases the Vm decreases to the point where the volume of the gas becomes more significant. However I don't see how this would cause z to increase above 1. Could someone explain that?