warmer air convection upstairs. warmer air holds more absolute moisture. also likely bathroom/showers upstairs. vent moist air outdoors during baths/showers and keep bathroom door closed run fan longer after finished.
recirculating range hoods also notorious (instead of exausting). clothes dryers not venting outdoors or having lint collections in the vent so can't properly vent notorious moisture loaders indoors.
can get a measuring device that indicates relative humidity can find combination display thermometers that also have RH. Might want to invest in a few, place in a few rooms..