My guess is the fact that some people get paid every other week. So they only have so much they can spend on games and what not. If there is an "Ok" ninja this event, but a good one next week, they know to save for that. But if there isn't anything better next week, maybe they are happy getting the "ok" ninja now, or spending on a decent event for items (lucky board). At the same time, if there is a Good/really wanted ninja next week, they know not to spend during this event.

From a business stand point, you also have to understand that most people won't spend on an "ok" event if they don't know what the next one is, because they believe (due to rumor or something else) that next week is better. I know I would have spent more at times if I wasn't told next week would be better... when it really wasn't.