There may be a connection between the families we don't know about, or several.
Even if not by direct union or by blood, but by in-laws or siblings marrying to an in-law family of the other branch, there was a chance of infidelity resulting in a child, and no one would ever know.
For example, as I'm sure others have discovered, TJ Mitchell lived in close proximity to his future wife when she was five, so he was most likely familiar with the Henleys for years before they got married. One of Lourana's brother's descendants married a Livings granddaughter. No Mitchell connection, but they were related by marriage and most likely the two families knew each other. I think TJ and William lived on the same street at one point.
And, for all we know, TJ dallied with one of Lourana's sisters or aunts. Or one of the daughters or wives of the families he boarded with before he went off to war. This is why I look at everything. Pages and pages of censuses, even after I've found who I'm looking for.
Also, when you say unaccounted for daughter, are you referring to the MC herself, or her mum?