A simple explanation would be extra scrutiny for the answer paper evaluators when marks go beyond 80. That might determine the difference between getting home/hotel at 5:00 P.M or sitting with the supervisor, painstakingly cross checking every answer. Picking answer paper evaluators are like picking people for jury duty in US. Its a lottery system for teachers across the board(In this case across India) and once you get selected, its mandatory to show up at the central evaluation center. The evaluations last something like 10-15 days.