Between the years of 1980 and 1990 the McDonnell Douglas company supplied at least 10 different variants of the Delta rocket family. Some of these were considered to be Delta rockets while some were Delta II rockets.
This became so complex that they invented Delta Numbering System just to try and keep track of it.
All this variety gave way to United State's Air Force Evolved Expendable Launch Vehicle program in 1995, with the goal of developing a more reliable platform for reaching space.
Why is there such a large variety in the members of the Delta rocket family? Some flew large numbers of missions, like the Delta 2000 series with 44 flights. Later on some became total disasters, like the Delta III family.
Were all Delta rockets essentially made to order for a specific mission? Did each launch come with some degree of customization?
The only alternative theory I have been able to come up with is that START treaty was put forward as early as 1982 by Ronald Reagan. Although it would not enter law until much later, did the discussion of this treaty leave McDonnell Douglas looking at the prospect of having far too many parts sitting around? For example, Wikipedia even states that the Delta 5000 was produced from spare parts