It could be a genuine mistake by the original writer, but I expect a textbook to have higher proofreading standards. Especially if this is a grade-school textbook (it looks like one), where you can’t reasonably expect the student to reference other sources to verify the contents, then I would expect the textbook publisher to put a lot more effort in to catching this sort of thing. And I don’t mean someone reading over it for typos, I mean someone who knows the field the book is written about, who can proofread for accuracy not just grammar. Genuine mistake or not, this is completely inexcusable.