This is normal bro. Don't take any tension.
When you put gear into R, D,S,L this is electronic system controlled by TCM and ECU and put load on engine to move forward and reverse manner. Hence rpm should decrease.
If you put in to P, its mean your car transmission is lock and unable to move forward or reverse. Hence engine is free of load and rpm should increase for proper working.
When will you service the throttle body? Also check the air filter.