Pressurising of the fuselage
stickyb,
If you increase the pressure differential between the aircraft fuselage and ambient air, the altitude within the cabin decreases, i.e. If the outside pressure was 14.7 psi (normal standard day) and you pressurised the aircraft by 0.1 psi, you are decreasing the altitude within the cabin. If you had some way of removing air from the cabin, and making it less than 14.7 psi, you would increase the cabin altitude.
Clear as mud..!