I've observed this in my car, and in the motorcycle (carburetor) that I had previously. When I start the car in the morning for work, the engine RPM starts off a bit high than the normal (+0.3 - 0.5 RPM) RPM. But when I shift to Drive from Park, the RPM settles and goes back to normal. This won't happen again for the rest of the day. So why does the RPM spike up in the morning?
Because the engine has got to its coldest overnight and the engine management system (ecu) increases idle slightly to provide a smooth idle - usually 100 to 500 rpm added as necessary.
In winter my 750rpm idle increases to 1200 rpm but drops down as the engine warms.
For the rest of the day, the engine has some residual heat from the previous use so the engine ecu does not need to compensate so much or at all.
Just for info, when we had manual chokes - the initial movement caused an increase in idle speed and further movement caused a change in the air/fuel ratio.
To add to @SolarMike's answer, the reason why your RPM settles once you put your car into Drive is because in automatic transmissions, what you're doing is engaging the transmission so that your engine can power your wheels. This is facilitated a torque converter - basically a fluid connection between your engine and the wheels that still lets your engine run even if the wheels are stationary.
The thing is, the larger the difference in your engine speed vs wheel speed, the more resistance there is, and more heat builds up in the torque converter. So instead of keeping your RPMs increased to warm up the engine, when you put it in Drive, the ECU assumes you'll start driving soon and it brings your idle RPM down to reduce heat in your torque converter as well as save fuel.
If you want to keep warming up your engine a bit, what you can also do is shift it into Neutral - Neutral is similar to Park in that your engine is disengaged from your wheels, which means your RPM can remain at the slightly increased level.
Entirely normal; it's called the "open loop" mode. The engine control unit is ignoring the MAF, MAP, O2 sensors output and enriching the fuel/air mixture to aid in starting the engine and bringing the coolant (and, therefore, the engine block) to operative temperature fast enough (in the meantime, a heating element inside the O2 sensor is bringing it to the sensor's operative temperature as well). After a few seconds, once a certain coolant temperature has been achieved, the ECU automatically goes into the "closed loop" mode, that is, it begins optimizing the fuel mixture by taking into account the sensors output, so that less fuel is wasted away and the emissions can be lowered. Less fuel also implies lower idle.
The "closed loop" mode may just enable itself by the time you've put the car into "drive", because the "open loop" mode is designed to last as little time as possible to reduce emissions.