The heating comes on earlier than the selected time because the Auto-Adapt function is on.
Auto-Adapt is an intelligent feature of Netatmo products that can switch on the heating earlier to reach your chosen setpoint temperature at the exact time indicated in your Home heating schedule.
Auto-Adapt can be turned on or off in the Netatmo settings: Settings > Operating mode > Auto-Adapt.
What is the heating anticipation function (Auto-Adapt)?
Updated