Isn't moisture a problem when measuring flour by weight?
Here is a post on the subject I found interesting.  http://www.genuineideas.com/ArticlesIndex/flour.html

The findings may not be definitive, but seem logical enough to be at least close.  Basically, my paraphrasing, they find the difference between completely dry flour and very humid conditions for most of us to be real but much less than that from volume measure.  They get more technical on some of their justifications than some might prefer, but generally claim that weight should not vary by more than 3% for most of us, and that given most home scales are accurate to about 1% and we classify that as precise measuring, 3% is not huge.

Guessing here, but I would suggest that other ingredients may also be effected at least in this same range by humidity and this effect is less than altitude variations, and that volume measures are likely effected by humidity which would allow increases in compaction as much, maybe ore than weight.  Given 3% as a rough guide, I think this fits into feel and experience areas and normal variations in cooking.  For those outside the "normal" band, desert and high humidity coastal type areas, having lived in both, yes, it is one more curse of trying to get consistent results there.