Past perfect is often used to denote something happened further in thee past than something stated in the past tense.
That is very often true, but not helpful in the example we are discussing:
Before they had gone more than a hundred feet, however, they turned a corner and ran headlong into a group of four soldiers.
The turning into the corner takes place before a hundred feet have been covered.
Quirk et al, quoted by suprunp in post #6 appear to offer an explanation, as does Tdol’s suggestion of dramatic effect in post #5.
Let’s look at some others:
[FONT="]1. He had left before I arrived[/FONT]
[FONT="]2. He had left when I arrived[/FONT]
[FONT="]3. He left before I had arrived[/FONT]
#1 is a classic example of the past perfect used for a past situation completed before a later past situation. However, unless we really wish to stress the ‘pre-pastness’ of the leaving, the use of
before makes the sequence of events clear, and it is possible to say, “He left before I arrived”.
#2 is another classic example of the past perfect used for a past situation completed before a later past situation. In this example, as
when does not make the sequence of events clear, it is essential to use the past perfect if the leaving took place before the arrival. If we were to say, “He left when I arrived”, we would be saying that the leaving took place at the same time as, or very soon after, the arrival.
#3 is the apparently paradoxical situation. Despite the past perfect, which suggests that the arrival took place before the leaving,
before tells us that the leaving took place before the arrival. Quirk et al provide the solution: the before-clause may be non-factual; that is to say, the arrival may not have taken place. I think that Tdol’s suggestion is also relevant: “the past perfect helps create a dramatic effect- setting us up for the next stage in the narrative”.
I think we may have to accept that this could be one of those situations in which we ultimately have to say, “That’s just the way it is”. The suggestions given by Quirk and Tdol help, but I would not like to have to defend them in a court of law. They do not sit happily with the idea that, in narrative, the past perfect is used for an earlier past. It is not so used in the sentences we are talking about, and those sentences are natural and correct.