Yeah, it's not uncommon these days for there to be an adult novelization and a "junior novelization." It's also conceivable that there could be an American edition and a British edition, published by different companies for different territories, using different writers . . . although I've never known that to happen.
Where it really gets tricky sometimes is when the movie changes the title. In olden days, this wasn't so much of a problem. You just put the movie logo in big type on the cover and the original book title in smaller type. But nowadays you really want to use the movie title so that search engines can find the book when people are shopping on-line, so there's going to be more pressure to change the title of the book to match the movie . . . at least on the tie-in edition.
In some cases, the movie title definitely wins out over time. Nobody remembers Somewhere in Time
as Bid Time Return
anymore, even though that was the original title of Matheson's novel. And I'll cop to using the movie title on every edition Tor has ever published . . . with Richard's permission, of course.