Sorry to revive this old thread, but I did come across some info on this subject on wk2jeeps.com. http://www.wk2jeeps.com/wk2_oil_indicator.htm
"...Based on the inputs, at a fixed number of miles since the last reset, the Powertrain Control Module decides on the severity of the operating conditions. If one or more of the conditions indicates severe use, the PCM sends a message over the bus so that the appropriate message can be displayed. If the PCM decides that the normal service interval applies, it waits until additional mileage has accumulated before sending out the request for a message."
This probably explains why some of you received a message at exactly 4000 miles to get an oil change b/c that is probably a set interval. If my light goes off, i'm bringing it in for more oil.