I've thought about this on more than a few occasions as I spent many years in Florida reloading in the garage. The powder manufacturers have quite an allowable range of moisture content to their powders. This directly affects powder density. But I would think it would not affect the total energy of the charge so that if the moisture content is not so high as to cause degradation of the powder, I suspect the only affect it would have upon ignition is to slow and cool the process slightly. This is all conjecture on my part.
I'd have to go back and look at some old notes but I recall leaving cases charged and open to the atmosphere for 3 days had no appreciable affect on performance, but that was back when the only rifle I had was a 30-30 and a sporterized Springfield. I do know, though, that varying density changes load density which can affect performance but normally will not be seen unless shooting longer ranges, say over 300 yds.
I think the take-away is that powder does gain and lose moisture but should not be an issue to the normal handloader. In the OP's case, leaving the original charges as initially thrown was the right thing to do. What was lost overnight was probably moisture which was replaced with powder.
One last point: We typically read humidity as "relative" which is a ratio of current air moisture content to the maximum it can hold at that temperature. Even if relative humidity is constant over a 24 hour period, absolute, or how much moisture is actually in the air, constantly varies with temp.
I'm a pragmatist, not a scientist so hopefully some of the more learned here will chime in.