Why are we still seeing mispellings (<-) in written works?

I just finished reading an awesome book entitled "Implementing Lean Software Development" by Mary and Tom Poppendieck. They preach some of the best practices in lean agile development for eliminating wasteful processes and eliminating mistakes. Yet on page 231 the word "manager" is spelled "manger". Manger is a word for a box that a horse or cow eats out of. I just wonder in this day and age how a publisher let this mistake go to print? Are they not using spelling and grammar checking tools? If they are, the software needs to be refactored to produce correct results. Maybe the publishers should read and incorporate the lessons from there own book!