I believe that weight, length and gauge of yarn are ALL rough estimates only.
Try spinning your own yarn and labeling it with definitive measurements. Sure, I can put an accurate weight on each skein, but as far as the length goes, it's an estimate. And the weight of this skein that is about 100 yards may be vastly different from another that is 100 yards. I run all skeins through a yarn meter twice and list an average length. But true length can be measured differently... even infinitely different. Do you want the length of a fully stretched and taught yarn, or the length of a fully relaxed, and not-stretched-at-all yarn, or something in between?
I have the time to carefully consider the weight, length and gauge of each and every hank of yarn I produce, but believe me, its tough enough for a little low-production one woman spinner. I can't imagine how tough it is for a big company to come up with dead-on accurate measurements for the thousands of balls of yarn they produce.
And as far as knots go, one knot is expected in every ball of yarn. If I don't find a knot, I consider myself lucky to have picked that particular ball.