Bid price or midpoint for a sell trade "Benchmark Price"

CFA Level III Portfolio Management
#1

In the CFA EOC question 10 page 54 Volume 6, it uses the midpoint as the BP for Q10/part two indicated in the answer. I am wondering why cannot we use the quoted bid price as BP since it is a sale of a trade?

#2

Anyone?

#3

From what i understand we are trying to calculate the Implementation shortfall which uses benchmark price or also decision price (for calculating delay costs under implicit costs).

#4

I noticed this. May be because average daily price is used instead of closing price. Often method.