It does look as though the other man on the page, William Henry Gerry, married Mary Ann Harvey (at least he later appears with a wife Mary Ann), leaving Sarah Jane Northmore for John. As has been said, if he was lawfully married to Sarah it may explain why there's no marriage found to Mary Ann Mills.
I am still unclear as to whether John or Mary Ann nee Mills have been found in any census, and where the expected c1840 birthdate for John comes from.