It was perhaps badly put, but the author is obviously aware of that, as shown by the paragraph immediately thereafter, in which the possible correlation of the

It was perhaps badly put, but the author is obviously aware of that, as shown by the paragraph immediately thereafter, in which the possible correlation of the variables is discussed. It is true that there is a certain amount of informality in the wording of the quoted paragraph, however. I agree that the author shouldn't have glossed over it. He does clarify later though, which makes the whole thing not so much of a big deal, especially as it's the very next paragraph.