MANFOD.
Perhaps a generalisation .
You don;t know how many economy passengers (if any) were upgraded to business to part fill the cabin.Just because there may be 20 in business class certainly doesn't mean they are fully paid business passengers.
We recently travelled economy and as the business class was so empty (not a MAN flt) we and 3 other couples were upgraded to business at no extra cost.
Didn't seem to go down well with the few that had paid the full business rate !
Nigel