Not sure if this might be the answer, but my understanding is that on the issue of the EASA CofA this will be issued by the CAA.
When you come for next year's ARC, you will be going through a maintenance organisation. They are permitted to extend the ARC up to two times, but only for those they themselves have issued.
Because the CAA have issued the initial one, they are not permitted to extend it, even though they may have done the work.
A neat bit of stupid bureaucracy