Just installed aprileaire 700 whole house humidifier. Furnace takes air from outside
Had to move controller (with sensor on back) farther away to where only in house air was moving through duct. In my house this was at the ceiling as the return plenum area was mixed with outside air and inside.. 

Sort of like a  "T" shape. Top Right side of T duct brought air in from outside, left side brought air in from return vents, and the middle (body of T/ Plenum) is where the air mixes together.. And of course outside air will have higher humidity, so it was messing with the readings. 

Hope this helps someone else one day.