I wonder if it might be related to the Whacking Day quest lines. Ignore for a moment the main whacking day quest line which gives you snakes upon completion of each task. And also ignore the quest lines that require premium purchases, Lugash and Ninja Homer.
That leaves five quest lines they throw in with not much benefit to complete - Gymnast Lisa, Duff Stadium, Bare-Chested Willie, Drederick Tatum and Miss Springfield. With five bonus level prizes, this matches up.
Complete one Quest Line for 30 Doughnuts, two for 75 Doughnuts, three for 150 Doughnuts, four for 200 Doughnuts and all five for 250 Doughnuts. Why else would you bother taking Homer off a Snake Hunt for 24 hours to visit Duff Stadium?
This is all speculation. Could be completely and totally wrong.
