Respected Mr Alan!
The same topic has been discussed on other English forum recently. Some learner of English has asked this question and received the following explanation:
"Sometimes the past perfect ... is used in a subordinate clause [such as "before I had finished my sentence"] beginning with before or an equivalent word, to indicate an action which should have preceded the action expressed in the main clause, but did not actually do so."
Let's look at this sentence: He left before I had finished my speech.
a. I started my brilliant (just joking!) speech at 7 p.m.
b. At 7:15, he thought: "What a boring speech. I'm leaving." So he (rudely) walked out.
c. At 7:30, I finished my brilliant speech and hundreds of people applauded wildly. (Just joking!)
If he had been a courteous person and had stayed for my speech, then I could have said:
I had finished my speech before he left. (But that DID NOT HAPPEN.)
Here is another good example:
"He gave his decision before he had studied the data."
He is not a good manager. Why? Because he HAD not STUDIED all the data
BEFORE he gave his decision. (That is what "SHOULD HAVE HAPPENED" -- but didn't!)
CAN this explanation be another interpretation of this structure?
I wish to know your opinions.
Many thanks for taking your precious time to take a look at this!