Why is there so much yarn left over?

Knitters often complain to their LYS that a pattern called for way more yarn than was necessary to complete the project. 

Occasionally when I worked in my LYS a customer would facetiously tell me it was because of the yarn industry conspiracy to get you all to buy more yarn.

The short answer as to why this happens is because as a designer if I overestimate yarn requirements some knitters will be annoyed. However, if I underestimate the yarn requirements all the knitters of that pattern will be furious!

The long answer is that knitters may use up more yarn working more than one swatch to determine needle size. They may work to stitch gauge and ignore row gauge. Row gauge differences can heavily impact yardage requirements in either direction. They may modify the garment in length. They may fudge the gauge and knit a different size because they can't get gauge.

Designers vary in the amount that they increase yardage to accommodate knitters. The increase varies from 10 to 20% on average. The other wrinkle is that if the yardage calculates to a tiny little bit over the yardage of a set number of balls we have to go up by one more. If you are substituting yarns you may be dealing with different yardages per ball than the yarn listed in the pattern. Requirements are often quoted in total balls as opposed to yardages which is a less accurate way of listing the materials required. 

We also know that many knitters have so many projects on their knitting list that running back to your LYS for an extra ball or two won't be possible by the time you get around to knitting your project.