The standard HVAC design is a single zone system with a single thermostat in the hallway of the main floor.
One thermostat in the main hallway cannot sense the temperatures upstairs nor downstairs; it can only sense the temperature directly around it.
Heat naturally rises, and thus temperature stratification (when it’s too hot upstairs and too cold downstairs) is unfortunately the norm, and has been for decades. Many think that is just the way it is; with iFLOW, it doesn’t have to be.