2 Conjectures
1 - Contrary to what Holywood would have us believe, a primary radar screen can be quite messy at times and much skill is needed to produce a valid interpretation of what one sees. Even more so at the fringes of the equipment range. It may well be the case that some radar operator is "not quite sure" of seeing an unexpected return around that time in a westward direction. Malaysian authorities are under a lot of pressure (at least, a lot of attention) to show some results and maybe shooting in both directions makes sense, but claiming that the plane turned back only to find it further down the intended track the next day would look bad, as would look bad to concentrate search efforts in the Gulf of Thailand and find the fuselage much later in the opposite body of water. Hence the ambiguity in saying that some things can be told, some can't.
2 - It was a very sophisticated operation dreamed about in some think room in some powerful nation-state at some point in time and then filed, to be used later when needed, so as to divert world attention from more pressing matters...