@Ce72 Most likely is that the thermal pad's thermistor is too close to the heating element, so reads higher than the rest of the bed surface when the heater is on (localised hotter temperature), then cools quicker than expected as heat dissipates through the bed. This tends to make the heating cycles shorter as they go on, causing the inconsistent behaviour, and long tuning cycle.
However, the tuning numbers you're getting are all pretty close, I'd think choosing the median for each parameter would be fine. If it triggers heater errors when heating normally, you can manually adjust it, see https://docs.duet3d.com/User_manual/Connecting_hardware/Heaters_tuning#manual-adjustments-to-the-heater-model-parameters
Ian