This method is still in use in some places, including the NBB, because of tradition.
David desJardins really believes it is the best method.
You play a bunch of boards and, being a perfect judge of how the rest of the field will play, conclude at the end of play that you expect to score exactly 60% on average against this field. Of course, you will score better on some days and worse on others, but on average let's say you are right and you will score exactly 60% if you could average over many repeated trials. Then the estimator above is unbiased if, when unbeknownst to you some other pair somewhere else in the room had a fouled board and scored average, you *still* expect to score exactly 60% on average, if the event were repeated many many times.
Or, when you deserve 60% in one group, you deserve 60% in a group of any size.