Yeah! I recall reading some of the stories that didn't even mention Miss Marple or Poirot until maybe halfway through the story. It was odd. These are your title characters! I've never looked it up, but I wonder if the writer thought them up as title characters or if it was a marketing thing. She may have just used them as a way to build up a series of novels; having recurring characters is a way to build familiarity and draw in readers.

It's just a unique way to approach it. Completely unlike their movie and show versions.