Refer to figure.
• True altitude is the vertical distance of the aircraft above sea level, specifically above mean sea level. 
• Indicated altitude is the barometric altitude displayed by the altimeter, which shows the aircraft's height above mean sea level.

Adjusting the altimeter's sub-scale setting changes the altitude reading. Decreasing the sub-scale value (setting a lower pressure) will cause the altimeter to show a lower altitude. This happens because a lower pressure setting moves the reference level closer to your vertical position, and vice versa. For example, changing the sub-scale setting from 1030 hPa to 1010 hPa will lower the altimeter reading, as the distance between the aircraft and the 1010 hPa pressure level is less than the distance to the 1030 hPa level.

When flying from a high-pressure area to a low-pressure area, the actual altitude decreases while the indicated altitude remains the same. This discrepancy causes the altimeter to over-read the aircraft's true altitude.

• Flying from high pressure to low pressure: actual altitude < indicated altitude (QNH)
• Flying from low pressure to high pressure: actual altitude > indicated altitude (QNH)

With the altimeter set to a QNH of 1030 hPa, if the indicated altitude is 8500 ft, but the local QNH is actually 1010 hPa (20 hPa lower), maintaining a QNH of 1030 hPa would mean the true altitude (based on a QNH of 1010 hPA) of the aircraft is lower than the indicated altitude by:

20 hPa x 30 ft/hPa = 600 ft

Therefore,

⇒ True altitude = 8500 ft - 600 ft = 7900 ft