maybe i can explain that-
in normal weeks each team normally has 1 game a week so the league average is more or less the actual league average in terms of income and expenditures but during the playoff week the 5th and 8th placed teams have 0 attendance(cuz they dont have any game).the same goes for the teams who don't have a home game during the playoffs or those who have been knocked out of the playoffs.but when the average attendance is calculated it would be like-
total income from attendance/total teams(definitely 16)
so as the number of home games decrease,naturally the average income would decrease