Auto-dim rear view mirror A4 Avant

I picked up my Avant from Audi at the end of last year.....one of the 'features' is documented as 'auto dim rear view mirror. I've been driving for 40+ years now but this is the first time I've come across and auto dim mirror....so please excuse the stupid question.   During daylight hours, the mirror is perfect and i can see everything clearly. As soon as it gets dark, the mirror goes very dark...so fully dimmed.   I would have thought I would clearly see out the mirror in the dark and when a car approaches from the dark, a sensor would tell the mirror to dim....but....it's just dimmed all the time at night......am I correct and it should be 'un-dimmed until a car approaches from behind, or is this normal, and once it goes dark, it is fully dimmed?   Tks