Marty
It's difficult to work out exactly the reasoning for the vast change to the search area. However, they have publicly stated it is because of new analysis of the radar data between the South China Sea and the limits of where it was tracked to.
I've only seen a portion of that data - obviously there's more, and whilst other nations have confirmed or denied that their radars also did or did not pick up MH370, we can only surmise that possibly they have been able to define more precisely some creative flying in terms of altitude excursions and turns, and possibly speed, which has resulted in less fuel available for the flight south.
A previous contributor commented that perhaps the initial phases of the deviation were more rapid but later the aircraft slowed a little to extend range. It had seemed that Inmarsat were inferring a regular flight level for the southward journey - perhaps that may have been revised also which may have affected the ultimate range.
I think if the final position is ever determined, there will be food for thought for months on this one!