ranco temperature is always off by 2 to 6 degrees

Ok I got a pretty big chest freezer prolly 15 cubic ft. I researched about probe and found that bobbym recomended putting it against wall of fermentor with insulation on it (i used a dog diaper). This gave me for example 66 degrees on my ranco and my beer was actually at 68. Now I added a little fan because I heard thats a good thing it made sense but now my temperature differs 6 degrees. I'm wondering what to do about this because it has really been tedious. I'm kinda tired of looking at ranco getting my temp gauge sanitized sticking it then repeat. Any recomendations further than I already have done?

additional information: I'm suspecting the fan pointing at fermentor might have thrown it off the extra 4 degrees. I recently taped the insulation tight around probe and waiting for results.

I use a separate indoor/outdoor type digital thermometer to monitor the fermenter with the probe attached to the side and insulated from the surrounding air. I have the controller probe in the air stream of the fan. I adjust set the controller according to what the thermometer reads. I don't worry about the air temp swings. I have the controller differential set to 5 deg. IMO, trying to use the controller probe for both functions is not the best way to do it.

Also, when making changes to the controller settings, keep in mind that it will take a very long time for the fermenting beer to catch up and stabilize. More than just a few hours usually.