This can be frustrating, but there's almost always a clear explanation.
Jumping readings for no obvious reason The soil sensors measure capacitively and are very sensitive to what's directly in contact with the probe rods. Common causes of fluctuating readings include air pockets around the rods, rods touching a drainage layer (e.g. expanded clay at the bottom of the pot), or a root that has grown around a rod. Check that the rods are fully inserted into the substrate and press the sensor back down evenly.
Different readings after moving or repotting the sensor If you move the sensor to a different spot in the pot, the readings may differ from before - and that's completely normal. Substrate isn't a homogeneous medium: water doesn't flow straight down but finds the path of least resistance, sometimes getting trapped in small air pockets. Different spots in the pot can therefore have different moisture levels.
If the readings are higher after moving the sensor, there's still plenty of water at that spot - wait to water until the app tells you to. If the readings are lower, more water had collected at the old spot - again, only water when the value drops into the yellow or red range.
Check the soil type in the app If you've changed the substrate, make sure the soil type in your plant settings is updated too. The wrong soil type can cause readings to be interpreted incorrectly.