Recently there was mention of the Court of King James of Scotland (think it was Scotland) and their ideas of what constituted a "marriage" which sounds very human but which lawyers and legislatures have fought about for centuries so this puzzle over Lucy Fitzpiers --
Lucy may herself have been a "wrong side of the blanket" child (in Olden Days, the "Fitz" did indicate that) or that her father had been
Such things as legally sanctioned marriage (or not) made a difference when it came to inheritance of money and land and social rank along with such points as political influence with the Court (the Crown)
And among the Royalty, calling someone a Royal Bastard was not an insult, it was a fact, in that the King (usually) had acknowledged the child and made some provision for it -- so too the siblings of the Kings and Queens, they were also Royal ....
And then again it might be errors have crept into the accounts