I grew up with Nancy Drew mystery novels, and it wasn't until I was well into my 30's that I learned that there never was an author named "Carolyn Keene." It was a series of writers (mostly men) who all stuck to the same Nancy Drew mystery formula. They were paid for their work, but they remained anonymous because the Carolyn Keene pen name had become a successful franchise.
Recently some authors are "partnering" with a famous person - for example Bill Clinton - and both get the authoring credit. What really happens is one person does the heavy writing while the other looks over his shoulder and makes a few suggestions. The author Anne Rice became so successful and synonymous with vampire and horror stories, that she adopted a different pen name in order to publish women's erotica stories that weren't about vampires. However the fans of Anne Rice books (the books marketed as vampire novels) became confused when she later published religious stories and biblical tales under her real name, Anne Rice. The vampire fans were turned off.
In the case of the Spanish authors, they're OK with partnering under the pen name of "Carmen Mola" because it's very difficult for young unknown writers to get published. When you hit on a formula that succeeds, you keep it going as long as possible. When the fans learn about the deception, I guess they feel confused and let down.