Wind chill is about cooling rates.
A thermometer cools down to the same temperature as the air, but your body tries to keep itself at a constant 37 deg C. If there's a -5 wind chill, your skin is losing heat at the same rate as it would at -5 in nearly still air.
When I worked in the Antarctic, we used to call it "bullshit factor", as the numbers on the temperature/windspeed charts then in use gave not very realistic, but impressive sounding numbers. I did crack the -100 barrier on a couple of occasions. The charts have been changed since, and I think it's more like -70 on the current scales. Still cold enough to get minor freezing of the skin round my wrists when working overhead, getting the ice off the anemometer.