In my experience, I have yet to meet a heat pump where the room temperature was lower than the setpoint temperature in a properly insulated room. It is normally higher. The only exception to that was when our replacement kitchen heat pump was installed incorrectly and was leaking gas - then it had to be set well above the actual temperature and stopped working at all a couple of days later.
I have also yet to meet a heatpump where the setpoint temperature matches the actual room temperature. The room is ususlly warmer than the setpoint, unless it is a particularly badly insulated and/or cold room.
Examples:
Kitchen Panasonic - setpoint 19 gives just under 22 unless it is colder outside, when setpoint 20 is needed for 22. But its Android app reports the inflow temperature reading as above the room temperature measured on a separate thermometer. For a real 22 and setpoint of 19, the inflow temperature is reported as 23-24. That is entirely expected, as hot air rises so an inflow sensor near the ceiling is always going to read higher than a thermometer that is lower for humans to read.
Dining room Mitsubishi Electric - setpoint 19 gives 21-22. When the outside is below 2, then setpoint 20 gives 21-22.
Blue bedroom Mitsubishi Electric - setpoint 16 gives 18-19. When the outside is below 3-4, it needs setpoint 17 for 18-19. When the outside is < 1, it needs setpoint 18 for 18-19. This bedroom, after the floor and ceiling insulation was installed, but without heatpump, would fall to around 8 on frosty nights. Before the insulation, it used to go down to 2.
Pink bedroom Mitsubishi Electric - setpoint 16 gives 13-14 on normally cold days. When the temperature outside is below 5 setpoint 16 only gives 12. This bedroom is particularly cold - after insulation of the floor and ceiling but before the heatpump, it would typically fall to below 4 on frosty nights.
So you just set the setpoint to whatever gives the temperature you want. The setpoint values are not calibrated, as that would have to be done with the heatpump installed as the circulation and insulation of the room affects the inflow temperature where it is measured at the top of the heatpump. The setpoint numbers are reasonably precise, but not accurate without calibration.
When the outside temperature falls low enough, the change in how fast heat is lost affects the room temperature and the heatpump needs to be set higher to compensate, as it does not see the change due to its inflow sensor still getting the hottest air in the room from close to the ceiling. Heatpumps typically only know two temperatures, the inside inflow temperature and the outside inflow temperature. They do their best to use that information to maintain a setpoint temperature, but without having a remote sensor at a mid-level height out of the airflow, they have no way to know the actual room temperature. So I think they do pretty well working from the fairly bad information available to them.
