These auction houses often set the estimates low to generate more interest of obtainability. The big houses also routinely produce prices that are truly head scratchers, as in, items sell for multiples of what they are available for elsewhere due to the brand name of buying from Sothebys. In this case, it could be that the book is just hard to estimate. I have no clue what a good price would be for a custom bound 1st/1st.
I would have thought about 1/3 of the realised price for The Hobbit and next to nothing for The Lord of the Rings with a questionable signature.
I can't comment on the books but I know when Sothebys runs movie poster auctions the prices realized are absolutely insane and often the same item can be had for a fraction on ebay that very day. These books are unique items. Movie posters, condition aside, are mass produced so there should be no inherent value to the Sothebys label. Who knows. FWIW I really liked the look of the Hobbit.