At first glance, it seems counterproductive for Kuat Drive Yards to have made two competing designs that were so similar. In reality, the two designs represent a faction conflict with KDY, between the Kuat and Fondor ship yards. Over the past six years, the design and management boards at these facilities have waged a quiet war against each other for the control of KDY. The typical weapons have been in- and out-of-house corporate espionage, competing designs, blackmail, and assassinations. One person who could tell you much about this war is the assassin/bounty huntress Angelena Deness, the Angel of Death, who has worked for both sides on numerous occasions.
Each side felt that the faction who's Star Destroyer design won the service trials would be able to take full power. To a limited extend this proved true, with the IXSD being officially chosen by the Navy. But, the Fondor faction sold the design for the ASD to the Rendilli StarDrive Corporation, who would be contracted by the Empire to produce a number of ASDs, and by the CSA to produce one.
This battle continued with the development of the Executor-class. Records indicate that the two yards were competing to finish the first of these ships, which would be designated the true Executor and given as a gift to Lord Vader. Both claim to have made the real Executor, with the other having made the Reaper, Grand Moff Kaine's flagship (according to Fondor), or the Aggressor, Admiral Roek's flagship (according to the Kuati). Who actually made the Executor is open to conjecture, and will most likely never be answered. Until it is, the KDY civil war will never be answered.