There's always a fair amount of "hysteresis" built into these systems so they don't keep switching on/off at short intervals. Once on they'll stay on for a reasonable time.
I've noticed my auto lights do come on when you might not think it necessary. I suspect it's down to the sensitivity of the sensor to different light content, rather like my "reactions" specs which will darken even when it's dull and cloudy, I think it's UV kevels which your eyes might not actually detect. Also you don't want the lights turning off just when you enter a well lit carpark or similar, or someone else's lights shine at your car.
Make sure the area of the screen where the detector sits is clean. It's probably working fine, just a bit quirky.