Thermostat set to 71

When I checked the heater on some cold morning this week, the thermostat was reading 69 degrees ambient, but with 70 degree setpoint the heater was not running.  Not only was it not running, it was cold, as if it had not run at all recently.

I changed the setpoint to 71 and the heater immediately kicked in.  Thinking that there might be something wrong with 70 degree setpoint, I decided to leave setpoint at 71.  Perhaps on some future cold night I can re-verify operation at the 70 degree setpoint.  But for now, 71 seems to work perfectly, tracking 69 degrees during the moderately cold nights since I changed the setpoint.  With no thermostat at all, just the heater thermostat by itself fell to 65 on a freezing night, but it was falling to 65 only with moderate cold and the thermostat set to 70.  So, mysteriously, the 70 degree setting doesn't seem to work as well, as if it were in some different range having different setback or anticipation, for example. Reading the instructions on Sunday, I saw no way the thermostat could be programmed to act different in different temp ranges.  Perhaps the two thermostats interact better with the 71 degree setting and the DeLonghi set to 1.5.