I think I managed to understand this use of 了 in Charles N. Li, Sandra A. Thompson. Mandarin Chinese: A Functional Reference Grammar. There, they argue the perfective 了 signals an event with a perfective action, which is regarded completely "bounded" either temporally, spatially or conceptually by the speaker. On the other hand, actions that are "unbounded" are referred as imperfective or durative. In English, for example in the sentence:
He was reading when his father came in.
was reading is an imperfective action (unbounded and extending in time) and came in is a perfective action (bounded and completely described in time).
In our sentence:
这个表快了五分钟.
This watch is running five minutes faster.
the action to be running faster is bounded temporally by five minutes, and marked as such by the speaker with the particle 了.
As a side note, we know 了 must be a perfective 了 and not a change-of-state 了, because it occurs after the adjectival verb 快 (to be fast) instead of at the end of the sentence.