General discussion of the perfective or aspectual 了
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 as completely "bounded" (i.e. with an end point) 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 ending in time).
An event can be bounded if:
Its temporal or spatial limits are specified. For example:
他年纪比我大了二十岁。
He is twenty years older than me.
二十岁 (twenty years) bounds the event.
If it signals a specific event and its direct object is definite. For example:
他在面包上抹了一点牛油。
He spread a little butter on the bread.
一点牛油 (a little butter) bounds the event. We note the use of 了 depends on the feeling of "boundedness" of the speaker within a context. The speaker might use 了 if he/she wants to emphasize the information in the definite direct object (considering it defines completely the event).
If boundedness is inherent in the meaning of the verb sentence. For example:
他去年死了。
He died last year.
死 (to die) is a bounded action with an end point.
If it is followed by another event. For example:
我吃完了你吃。
After I finish eating, then you eat.
我吃完 (I finish to eat) is bounded by the subsequent event 你吃 (you eat).
A bounded action might or might not be a completed action. However, many perfective events refer to past events. Ordinarily, unless context makes clear a different time, a perfective 了 is understood to refer to past time. For more information, check the corresponding chapter of the book.
Note: The perfective or aspectual 了 should not be confused with the sentence-final or modal 了, which occurs at the end of a sentence and marks a change of state.
Our example:
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. In this case, the action might or might not be completed (the watch is not necessarily readjusted).