Pondering qualities of yarn

The impetus of this post is a scarf I'm close to finishing; it's two colours of yarn in Patons Classic Wool; first one colour held double, knit to the end of the two balls, then the second colour, done the same. One would think that this would make two equal halves of colour, yes? Not so. The yardage of the second colour is less than that of the first. Grr.

And this made me think: why? WHY? And that made me think that with most yarns, the same base yarn dyed in different colours can have very different hand (that is: texture, softness), and it seems that there can also be a yardage variance based on colour, which seems odd.

So, putting these odd questions out to the spinners and dyers... why are these things so? I mean, I KNOW yarn. The yarns I use frequently, and those we sell in the store, I can identify by sight and by touch. But this variance is something I don't quite have the knowledge about, and this annoys me somewhat.

Please to share your edifying, enlightening information in the comments. Or, by chance if you are Brenda Dayne (!), I'd LOVE to hear you get into these oddments of yarn ephemera on a future Cast-On.

Happy knitting!