Cameron has been dragging his feet on the promised Abyss/T2/True Lies 4K rescans for years; remember how long it took for T1 to get rescanned? (Though isn't that Cinemax transfer of Abyss "tealed", implying that it may be Cameron-supervised?)
Like dvdmike, I am of the belief that all the Blu-rays are based on one (2K?) scan done some time in the 00s (I also assume for the Extreme DVD), because they all look more alike than different.