Rain sensor logic?

[Price]

It was raining hard recently here in VA and I've noticed a strange thing in our new 08 X3: if you have wipers in auto mode and it's raining, they won't resume operation if you stopped and turned engine off and then back on (say you stopped for gas). Your windshield can be all covered with drops but wipers will be motionless untill you actually move the lever down to OFF and then back up to AUTO, only then they resume working.

I have rain sensor in my other car (LR3) and never experienced such a behavior. What is the sensors' logic? Is it reacting only to a difference, i.e. if you windshield is covered with drops already before you turn on ignition, it won't activate the wipers?