First, everyone involved is spinning the news so that they all look good. Second, it's important for Lionsgate to get production on the next installment started sooner rather than later, with Lawrence heading for a January "X-Men: First Class" sequel start date for Fox. (More details on that negotiation here.)
The questions Lionsgate had to answer were these: Who was really responsible for the success of "The Hunger Games?" Who did they have to keep and at what cost? Thus, how much were they willing to spend on the sequel?
So how crucial was it to the future success of the "Hunger Games" franchise to bring Ross back? He contributed a great deal to not messing up the move from page to screen. Like "Harry Potter," the author, Suzanne Collins, stayed involved and had the smarts to rely on one producer who she trusted--studio veteran Nina Jacobson-- to see the project through to completion. They selected Lionsgate as worldwide distributor. And they all chose experienced filmmaker Ross to direct.
On "Twilight," Summit was eager to move forward quickly and basically made it impossible for Catherine Hardwicke, burned out from the first film, to return for the second on an accelerated production schedule. Summit did not believe that Hardwicke was essential--they thought they could build on what she and writer Melissa Rosenberg had established, keep the writer and move on. The second "Twilight" directed by Chris Weitz was the most rushed and weakest of the series. But it was not a fatal error. They got away with it.
In this case, Ross is a more powerful player, and was making creative and money demands--as anyone would do in his position-- that would inflate the cost of the sequel. Again, with Jacobson and Collins and the original creative DNA to build on, Ross had done the heavy lifting and the Lionsgate players felt they could move on without him.