I thought that was supposed to be because of the nature of the event - it means that the cultists have been covertly building up an underground following, which when it goes overt, has an effect on the faith of the province, even if the defenders manage to defeat them soon afterwards.
I wonder if it can happen on turn one and dominion-kill someone who took Unluck-3...
We probably would've heard if that had happened to anyone here...
PvK