Refer to figure.
We know the following about pressure and temperature:

• When an aircraft is in a lower pressure area, the altimeter will overread. The aircrafts true altitude will be lower than indicated.
• When an aircraft is in a higher-pressure area, the altimeter will under read. The aircrafts true altitude will be higher than indicated.
• When an aircraft is in a colder airmass than ISA, the altimeter will overread. The aircrafts true altitude will be lower than indicated.
• When an aircraft is in a warmer airmass than ISA, the altimeter will under read. The aircrafts true altitude will be higher than indicated.

Since we fly on Flight level 120 (1013,25 hPa on our altimeter) and we don’t change this setting, our indicated flight level will not change.
But normally our actual flight level will change if we have non-standard conditions.

Our local QNH is 1013.This is the same as according the ICAO standard atmosphere.
So, until so far it is logical why the true altitude also indicates 12 000 feet.However, we have to realise that the temperature variation from ISA also affects our true altitude.
In this case because true altitude = indicated altitude (and we already know the QNH value is according to ISA)we know that their cannot be any temperature deviation from ISA.
True altitude (TA) would have been higher than indicated altitude (IA) if the temperature was warmer than ISA, and TA would have been lower than IA if the temperature was colder than ISA.
And thus, the average temperature is the same as ISA.