Doesn't mean it couldn't be sent somewhere else. It is a spaceship after all, warp currents etc aside, it can go wherever it needs to be.
Look, if a one of a kind car is built in, say, Italy. Would that stop said car from being driven beyond Italy's border?
Or anywhere else for that matter.
Ships in BFG get moved around with some regularity, a contingent from Battlefleet Gothic being sent to Cadia during the 13th Black Crusade for example. Should the Imperium win that conflict, whose to say that any vessels from that contingent be ordered to remain in the Cadian sector to cover losses there?