There are "obsolete" and "upgraded" versions of many Dropships.
The real life background is that the original dropships were designed without fixed construction rules (or at least without published ones, I remember that I - and probably many others - reverse engineered the rules for the most part back then).
When the construction rules were later codified and released, there were some discrepancies, so the old, now formally incorrect designs were grandfathered in as "obsolete" older versions. E.g., the old Fury had 200t cargo space, the new design has 474t.
DS types first developed in the 3050s and later exist only in a single version.