Well, considering how "everyone" (including nobility) ignores the existence of children who died at or soon after birth, it's not so terribly surprising that the official biographies state 5 sons.
It's also not surprising that Samuel and Minnie would informally adopt / claim Pauline as their own. It's what people did. (Hänne had already lost a son, in 1882, and, as she married a second time in 1892, it is easy to imagine that she lost her husband soon after the baby and things happened, perhaps through grief and seeking solace. It is also always possible that Pauline was legitimate, born just before, or soon after, Hänne's first husband died, and she just could not cope with a child so soon after the double loss. Family used to step in for those situations, too.)
I think it's fantastic that Manfred has been confirmed as a 6th child; and equally fantastic that Pauline is conformed as niece, not daughter (1930 census), even though she bore the Marx last name.
I've added the 1900 census to Samuel and Minnie, with a Research Note.