I had two coworkers who were diet-frienemies. One followed Atkins, and one followed South Beach, they were constantly bickering over which was the "best" plan, yet they were eating virtually identically.
Every diet writer wants to tradmemark their plan. It has to be unique enough to be published ("just do what this other book says," isn't going to get you a book deal).
I think most author's truly do believe they have a unique perspective, and something new and meaningful to say, but all that being said many are reinventing the wheel. Some are putting a new spin on it, or explaining the principles in new (not necessarily, but sometimes better) ways.