Air density doesn't have time to have an effect when the chrono is set within a few yards of the muzzle.
Some powders are temperature sensitive, and some are more stable. A variance of 1fps/1*F is not uncommon.
Here is an experiment
http://www.shootingsoftware.com/ftp/...%20Factors.pdf
In my own experiment, I have seen as low as 10-15 fps difference for 90F temperature difference using Varget and H4350, but it may be load-dependent.