I'm not sure exactly how it works with knitting books and magazines, but I would imagine it would be largely the author's responsibility to ensure her/his book was error-free.
Editors don't actually "edit" the book...they make the business decisions about what to publish and when and how much to invest, and how to promote the book and what the cover will look like and which layout to use etc, etc, etc.
Copyeditors/the author are usually responsible for making sure that what is published is what the submitted manuscript says.
I work in textbook publishing...we hire math professor to check the accuracy of the math in our books...the text, the examples, the problems, the answers.
But there's only really one right way to solve a problem. Maybe you could use a different technique get the right answer, but there will only be one answer. With a knitting pattern, some people interpret abbreviations differently. Sometimes, a designer will have something set in their mind and not make it clear/not notice that what is written isn't what it should say. Their test-knitters might be so familiar with them that they understand the missing instructions, or dont' notice errors. It's not until the pattern is out there that errors are revealed sometimes. :shrug:
I think it's annoying, but there's only so much time and money knitting publishers can invest to make sure that their patterns are accurate. At least these authors have made the errata available--nothing worse than finding a pattern with mistakes and no answers on how to fix it. :!!!: