Shunter. In the example you give of QNH = 1020 assume that you are flying at an altitude of 5000ft. Resetting the altimeter to 1013 will decrease the reading on the altimeter by 210ft (assume 1mb=30ft) therefore the altimeter will be reading 4790ft. If the aircraft is now climbed to FL50 its altitude will be 5210ft.
Therefore if the QNH is 1013 or greater and the altimeter is set to 1013 to fly at flight levels the actual altitude of the aircraft is greater than indicated. The converse is true if the QNH is below 1013.
Assume that the QNH is 963mb and the aircraft is flying at an altitude of 3000ft. Changing the subscale setting to 1013 will make the altimeter read 4500. If the aircraft is climbed by 500ft to fly at FL50 is actual altitude will be 3500ft.
In the example above with QNH = 1020 FL50 equated to an altitude of 5120ft whilst with a QNH = 963mb FL50 equated to an altitude of 3500ft.