05 911 Oil question

I have an '05 911. I check the onboard computer and the oil level is at max. When I am driving and stop at a red light, the oil gauge shows that the oil level is below 2.  My car only has 12,000 miles and I do add oil to my car. Minor maintenance is at 20,000 miles. Why is the service now light came on? How do I get rid of the service now light? Also is it normal for the light to come on and is it normal for the oil level to be below 2 when the on board shows that the oil level is full?
 
Thanks for your help.