They are different because the original 3 volume boxset is replaced with a newer 2 volume slipcase. This link might answer your question:
The Great Rift has been updated!
Now in an attractive two-book slipcase with three giant poster maps and a new chapter covering adventures in this vast expanse of space, it has been completely updated with stunning new art and the current Traveller formatting. Perfect for your Travellers to dive into a barely explored region of space.
Those of you who purchased the previous Great Rift box set will find that your PDF files on both our website and Drivethru have already been updated to the new edition.
Everyone else can grab their own copy right here...