Imagine if you left your aircraft overnight reading 0 ft at a pressure of 1012. Overnight the pressure changes to 1011, so the indicated elevation goes UP by 30 ft. So, to restore the zero reading you have to change the subscale reading from 1012 to 1011. That's the way I try to remember it - in re-setting the subscale reading to a lower value you are compensating for a reduction in air pressure, and this reducing the indicated elevation.
Hope that helps
Peg