Why does the screen brightness reduce once the screen turns on?

Whenever the screen turns on, be it when booting up or when waking up from sleep, the screen brightness comes on as per the set brightness levels and just after a second, the brightness drops by about 20% and stays like that. If I put the system to sleep and again wake it up the same thing happens - the brightness comes on as per the display settings for a second and then the brightness drops by about 20% and stays like that. This iMac does not have an ambient light sensor so what could be causing this issue?