ISD is not the date the hull comes into service, its the date that particular version of the hull came into service. At least that is the canned answer.
So the Xorr might have been around a long time with different weapons, engines or armor. Those have all gone to the scrap yard or refit dock, and as of the date specified we only have the current version.
Allows Mongoose the flexibility to go back and do an 'early era' for races as well as adding on to the end of the timeline.
Ripple