Can we take a step back and acknowledge that GW has released their fifth codex in 8 months? Or have we already forgotten that it used to be only 3-4 books [fantasy included!] per year?
We're going to have all armies with the same edition and design philosophy, at the same time, for the first time since the skeletal armylists in the 3rd Edition Rulebook. That's awesome.
Rules in a codex go through lots of changes during testing, and each change makes the document need to be updated in multiple places. It's easy for something to slip through [hence why the Summary is notorious for incorrect stats, as it has to get updated the most]. At some point they have to stop editing and send it to the printers. It doesn't magically just turn into a book, it takes time. During that time, they have time to notice additional errors that made it into the final print, and then release an errata correcting it as soon as the book is available.