I think the algorithm may differ a little, all of them give a slightly different result, usually, sometimes it may be the same. BoS's premium SAM gives you a comparison between different years, what you may have received in eg. 2005, 2006 etc... not 100% sure on this though.
They cant predict accurately anyway, until the final hsc marks are out, putting in raw trial marks and other exam marks what give you an accurate estimation.