The release of information to the public has NOT been in real time. Australian Naval assets were on their way to the search area 26 hours before it was announced, satellite information has been screened and kept secure.
All of which is quite appropriate and quite understandable.
I suspect the original Inmarsat 40 degree arc is true. But everyone was very dogmatic that was all the information available. As far as the satellite was concerned, the plane could have been anywhere on that arc at the time of the final ping.
If the Inmarsat data had been refined by other means, and if the US needed a cover explanation, who else would they turn to other than the UK?