Ok, this may be an unpopular opinion, but I'm throwing it out there anyway. I think it is possible to over-think what is going on. With any unit, I do believe there can be some areas that could vary slightly from other areas, but I personally don't think its that big a deal. With forced air, the temps should vary less throughout the unit, obviously. I guess I just don't trust the thermometers at all any more. I don't think any of them are accurate enough to spot check that precisely. Just watch the air cells and adjust temp and humidity accordingly! Don't get me wrong, I know a certain range is desirable, but I think we tend to go overboard sometimes, trying to make it "too perfect". I will add that I guess its possible if your heating element is failing, you could have big temp swings, so I guess that is something to consider.
(and the reasoning behind my theory... I had a 48 hour power outage due to a snow storm. I didn't get a generator hooked up for 18 hours! Outside temps got down below 0 F. I have a wood stove, but my incubator still got down to 65 degrees before I got it plugged back. And I only lost one duck. Well, I lost another 3 during the final hatch because they couldn't get positioned properly. This could have been a result of 18 hours without heat, but it can happen any time. Point is, the little boogers are more resilient than we give them credit for! So I'm not fretting over one or two degrees any more!) JMHO