If you mill 120 grams of berries, do you always end up with the same amount of flour (weight wise, not volume)?
yes because otherwise there would have to be "something" left behind in the grinder/mill and there isn't. However my grain mill kicks out fine dust whilst in operation which I suspect is normal and that dust must have a weight so therein will lie the difference though it is probably negligible. Never weighed it though.
It depends how you sift it. If you start with 100 grams of berries and sift out 20 grams of bran, etc., you have an 80% extraction flour.
That is approximately the sum total of my milling knowledge.
14 years experience with a mill tells me this is true. Every time. I mill about 1850g at a time and there is a loss of +/- 4g due to the fine dust (agreeing with elpanadero). I don't ever sift so I can't speak to a possible loss there.