I can think of two things....1 when your engine is cold it idles a bit high so that the battery is sure to get recharged after being drained from starting, ( a temperature sensitive voltage regulator also helps to do that). OTOH when you say "until the car stops", that reminds me of the way my engine behaves while driving. Saying your going 65 and decellerate moderately for an offramp....the way i understand it to work is that (keep in mind an automatic transmission is ALWAYS connected to the engine while in gear) you hit the brakes, the TCC unlocks, and the comp is controlling the idle of course but only drops it a few hundred rpms such that if you were to hit the gas again and accelerate there wont be such a thump or jolt to the drivetrain and car, making for a smoother performance. Only when you get down to <5mph or so does your car go to its lowest programmed idle 700-900rpm at operating temp for most. That'* my .02 but this is what ive observerd, and my bonny pulls like a motha and shifts like a dream.....
1999-2007: ___1988 Bonneville LE LN3 (Vin C) @ 273,000 miles
and still going strong (<-Faded sun fried grayish ie needs paint)
2007-Present: ___1993 Bonneville SE L27 @ 210,000
Speeding around So Cal (<-Beautiful Forest Green)
Mods: Sylvania Silver Star headlamps. Remote start system with pageing device. K&N air filter. Tranny Cooler. High-flow cat with Magnaflow muffler. Full Class II hitch setup.