Air density has a lot to do with it. Barometric pressure, Humidity and temperature changes things when it comes to reloading. This has been proven many times with Benchrest shooters. That is why they will load at the bench sometimes when they are shooting. Most winter air is drier. Even in a controlled environment like your house the air is drier in the winter than the summer. Moist air is denser thus the pan weight is at least a tenth different.
Now for a moment you would have to disregard the weighing of the charges. That is why they do not do it. That is why #27 on their powder charge will be used when say #30 is not grouping well.
But let's go back to the answer. Air can be heavier (because of moisture) and will settle in the powder also. Doesn't matter where you live or what kind of HVAC system you have.
For example if you left the lid off a pound of powder, even with a slight amount of humidity it will pick it up, and the weight will be different than if it was very dry, and the lid was only off for a moment and then replaced.
If that was the only factor involved then this would most likely not show up. But like I said before, their is a few factors involved. I agree with stopping fans or cutting off the air vent and letting the scale warm up, and calibrate it EVERY time I use it.
I have Harrel and Redding powder charges, along with my RCBS Chargemaster, and a couple of scales. Between the combination of the three, I can get as close as possible to what I want. This makes a difference in say long range shooting when near max or max loads are used. Why in the heck would I use different options? Because the RCBS is only within a 1/10 +or- So you could be off more than you think.
I have noticed the pan weight being different and have noticed handgun loads being slightly different when loaded, only using the RCBS Chargemaster by itself. I usually throw the first few charges back into the powder chute. To me they are sometimes off a little.
FWIW: Many years ago I noticed a Barometer and temp and Humidity gauge in this friends loading room. Asking him about it, He shared that the info was recorded each time he loaded and results where recorded also. This info was kept in a notebook and in turn helped him to produce much better loads for what he was going to be doing.
If you are not loading max, or near max loads and you are loading a set or standard load for ONE gun, then do not get overly concerned.
But if you are loading for some really accurate ammo then it is going to involve more than just this one thing. As you probably already know....