Yarn weights - a question

While working on the Shoulder Shawl in Cherry Leaf Pattern, I have been puzzling over page 176 - yarn weights. How do the determiners determine whether a yarn is superfine - #1, or fine - #2?

The most puzzling is why a hand-dyed 20/2 silk is a #1, while a 20/2 hand-dyed silk is a #2.

I know different fibers weight different amounts, so you can't go entirely by yards per gram, but Alpaca with a Twist Fino is an alpaca/silk blend at 875 yds/100 g. Kidsilk Haze is a mohair/silk blend and has 916 yds/100g. One is classed as a 1, the other as a 2.

I guess it's the mohair blend classifications that are confusing me the most. They look so much smaller than the rest of the #2 yarns, and have more yards per gram than some of the #1 yarns, and it seems like they should be interchangeable without much change in gauge with a #1, especially in openwork like lace. But substituting a #2 sock yarn for Kidsilk Haze just seems plain wrong, weight-wise as well as several other ways.

What are your thoughts on this?