It's all about thermal mass. If the probe is swinging freely inside your chamber it'll warm up inside quicker than if it is making contact with a keg, carboy, or other liquid.
If you left it on a 2 degree differential and swinging free the compressor will kick on and off much more often than you want.
At a 6 degree differential, the serving kegs and bottles in your fridge should even out at whatever temp you've chosen, or a degree or two colder. I have my fridge set to 38, so at 38 it'll kick on and drop to 32/33, and slowly warm back up *ambient*, but the liquids stay much closer to 36-38F.