Why an Astute submarine took 13 years to build

Why did Barrow take 13 years to build the Astute submarine being discussed?

Changes were made twice during construction, and the two Astute batches differ substantially.

Changes made twice during construction account for the long build in this explanation. The two Astute batches differ enough to resemble separate classes. The question does not clearly identify the submarine. That uncertainty prevents tying the 13-year figure to a securely named boat.