Lack of proofreading plain and simple.
Funny how many folks don't proof read anymore (including 'yours truly', on occasion). A lot of people figure spell checkers are sufficient, when they're really not, at least for professional work. Years ago, I was reading though a series of reference books by Norman Friedman, covering the design history of several types of US warships. You could tell when he stopped using a proof reader - the number of errors went from just a handful per book, to dozens. My favorite typo was a reference to "Admiral Normal Scott"...
“Lack of proofreading plain and simple.”
I don’t think so. Whomever wrote that wouldn’t have caught it by proofreading. If he were speaking he’d say the same thing.