The abnormal cells can develop that quickly - I think what Marionravenwood meant was that they would not progress to cancer that quickly.

The idea behind the less-often screening is not necessarily that it's pointless because you wouldn't find abnormalities anyway. The point is that most abnormalities will resolve themselves given time, and that less screening is safe (because it takes time for abnormalities to become serious enough that immediate treatment is necessary) and helps you avoid unnecessary procedures.