***** NOT A TEACHER *****
I cannot explain why, but I can offer some advice from one of my favorite books:
“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 a personal experience:
Many years ago at 10:30 a.m. I told some people: “I think Mr. X will return tomorrow.” The people then went home very happy.
Then at 11:45 a.m., I called Mr. X who told me: “I will NOT be returning tomorrow.”
I told Mr. X: “OMG! I told them that you would return tomorrow before I HAD SPOKEN with you.”
“had spoken” refers to an action that SHOULD HAVE PRECEDED the action in the main clause. That is:
I had spoken to you BEFORE I told them that you would return tomorrow. (But that did not happen.)
I know this is all very confusing. Here is a good example from that book:
“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!)
Walter Kay Smart, English Review Grammar.