As far as I can tell it was made up after the fact in very much the same way as the name 'Jacobite Trilogy'—by people trying to describe how these books relate to each other in the language of modern book series that didn't really exist when these books were published. Certainly my copy of Kidnapped doesn't say 'Book One in the David Balfour Series' on the spine!

(Thinking about older ways of describing related books, I wondered if this might hypothetically happen to Waverley if anyone posted fic for it. In fact four of the Waverley Novels have fic on AO3, all with separate and unconnected canonical tags. Though 'Ivanhoe - Walter Scott' is synned to the canonical 'Ivanhoe - All Media Types', which seems rather bizarre).