Keep in mind also that even if you've set your altimeter subscale to the correct local pressure setting (QNH), the altimeter won't necessarily indicate your actual height above mean sea level if the temperature is different from ISA (as it almost always is). If it's warmer than ISA, your actual (geometric) altitude will be higher than the (barometric) altitude that your altimeter indicates. If it's colder, your actual altitude will be lower than indicated, hence the need for cold temperature corrections to altitudes on approaches.
Edit:
aviationlover, may I hazard a guess that you fly in a fairly warm climate?