I have two sets of these and in both cases The Two Towers seems the worse for wear than the other two books, this seems to replicated in other copies for sale. You can see this in a picture from a current ebay sale below.
Deagol does not mention it in this article that I can see.
I am interested in any ideas as why one of the three books tends to have suffered more over time than the others?
Don't really know why the spine of TT is very prone to fading (difficult one to figure out) but it is definitely an issue.
Virtually all the ones I have seen for sale over the years have had the spine of the TT faded. I think it was about 8/9 years before I found a nice original set with an unfaded spine (and not a completely outrageous price - still was quite expensive - just not completely outrageous).
Sorry to be of no absolutely no help with your question. Don't even have a guess to venture!
The buckram is more prone to delamination from the underlying board on the TT as well. Presumably there was just a different batch of materials used for some or all of the TTs, which is less durable. I can't see any other explanation - It is either process or materials.