Inside of one room, it is frequent to observe difference in temperatures between two different measuring devices if they are not located close to another.
The temperature and humidity displayed in the Home + Control app are measured by the Netatmo Smart AC Controller. To make it as accurate as possible, be careful not to prevent the air circulation close to the product and to not cover it (curtain, coffer, voluminous object...).
Your air conditioner also has sensors that allow it to know to regulate the temperature taking into account the orders received from the Home + Control app.
Those sensors are not synchronised with the Home + Control app. Therefore, it is possible that a difference can happen. In this case, the sensor of your AC will stay the master and will be the reference to indicate to your AC when to turn ON and OFF to reach the right temperature.
The temperature calibration is not available in the Home + Control app.