You may find that your thermostat continues to call for heating/cooling even when your set point has been met, or that your heating/cooling is turning on earlier than scheduled. This is due to the ecobee's Smart Recovery feature.
The ecobee continues to accumulate data, taking into account the outdoor weather and the efficiency of your system in the past. It then uses the data it's accumulated for Smart Recovery, which prematurely heats or cools your home in anticipation of a transition in your schedule. The higher the difference in set points between these two programmed Comfort Settings, the earlier/longer the system may run to reach the target set point.
For example, let's say that your set point during your scheduled Away period is 78 degrees, while during your scheduled Home period, it's set to 72. Using the data it's gathered, the ecobee will turn on your air conditioner before your schedule shifts from Away to Home so that by the time your Home period comes into effect, the temperature will already be at 72 degrees (as opposed to waiting until the schedule changes and then trying to bring the temperature down from 78 to 72).
If you would like to disable this feature, you can do so by going to MENU > SETTINGS > PREFERENCES > SMART RECOVERY COOL/HEAT and switching it from "enabled" to "disabled."