That is fairly normal as a ship in refit or long term maintenance will be used as a donor based on supply time, unfortunately the days of spares in store for "just in case" have long gone, it is different to Leviathan which was never to be commissioned.
The big problem with no "just in case" is that when it is needed it may well be obsolete and you then need to procure a substitute that A. fits and B. doesn't cause problems, this can cost more than buying for store, with only two ships in class buying for store tends to be more expensive as there is no history of MTBF and in the carrier's case it could be 20 years or 5 years