Halfway through the Sister Fidelma series, Sister Fidelma renounces the religious life and starts referring to herself as just plain Fidelma.
In the Demon Headmaster novels, the title Diabolical Mastermind is only a school headmaster in the first book, though he's referred to as the Headmaster throughout because that's the context the heroes first encountered him in.
After book one, The Boxcar Children spend more time solving mysteries than encountering boxcars. They got the name because they lived in a boxcar for a while, but it sticks after they don't live there anymore.
They do keep the boxcar as their hangout spot on the property where they live but it's still a stretch.
Several English translations of The Phantom of the Opera translate the French "fantôme" as "ghost" within the text but, understandably, don't change the widely-known title; thus, the eponymous character is never actually called "phantom of the Opera" but "the Opera ghost."
The Ranger's Apprentice series, after Will graduated from being an apprentice to being a full ranger.
The Foundation Trilogy has an in-universe example. At first, "The Encyclopedia Foundation" was a N.G.O. focusing on the publishing of a compendium of all human knowledge. While they did eventually do that (sort of), the Foundation focused more on the Seldon's plan, and became an empire.
Inverted with Francine Rivers’ The Mark of the Lion trilogy, in which actual marking by lions doesn't feature until the very end of the first book, and doesn't feature at all in the third. (Though it could easily be inferred to be an important metaphor, what with Jesus being referred to as the Lion of Judah.)
I, Robot. Its title was borrowed from an earlier story by Earl and Otto Binder. Not a single story in Asimov's book is told from the perspective of a robot. Even better, Asimov is reported to have detested the title, which was forced on him by the editors. Also, an in-universe example shows up: USR, the robot manufacturing corporation at the center of the plot, continues to call itself "United States Robotics" long after the United States have ceased to exist as a country.
In the Shannara series, half the titles forget that Shannara is not the world, but a historical figure. This reaches its nadir with The First King of Shannara, which is about said historical figure and might better be titled King Shannara.
Since the books follow the exploits of the descendants of Jerle Shannara, it could be argued that he is the first king of the Shannara line. But all the titles would make more sense if "Shannara" were replaced by "the Shannara bloodline" or "the Shannaras" or even "the Ohmsfords" since Jerle's family spends most of history with a new surname.
Played straight in the most recent series, titled "Genesis of Shannara" and "Legends of Shannara". They take part long before the historical figure even existed and the only "genesis" in the first series is a new world.
In the Doom series, the third and fourth novels, Infernal Sky and Endgame have fewer and fewer elements of the video game they're adapting.
Similar to the Hitchhiker's Trilogy, Robert Rankin's Brentford Trilogy currently has nine books.
In the Rainbow Magic series, only the first seven books dealt with the rainbow.
Reversed in Dinoverse. Dinosaurs are involved from the start, but it's time travel, not The Verse-y at all. It's only in the last two books that an alternate universe called Dinoverse comes up.
In Vampire Academy, the second, fourth, and sixth books take place primarily outside of the eponymous St. Vladimir's Academy.
The fourteenth book of The Morganville Vampires spends only the first and last chapter in the city of Morganville, mostly spending its plot near MIT.
In the Bible, the prophet Samuel doesn't appear at all in his second book because he died in the first.