Conventional thermostats work thanks to a simple mechanical reaction between the unit and room temperature. This system lacks precision and causes variations of more or less 2 degrees Celsius from the desired temperature, which can mean a difference of up to 4 degrees Celsius.
As for electronic thermostats, they react to the slightest change in room temperature because it is continually measured by a detector. Variations being more or less 0.5 degrees Celsius, the temperature is more stable since the maximum difference does not exceed 1 degree Celsius.
With a programmable thermostat, additional savings can be had if it is adjusted to reduce the temperature automatically at night and while absent. For example, a reduction of 3 degrees Celsius in an 8 hour night brings a reduction of 4.5% of the cost of heating, a small change which can translate into significant savings.
Source: The Guide to Ecological Options, APCHQ