When comparing to the rational choice, I did what I do in real life: added a safety margin. If anybody tells me there is a 10% chance for a project to succeed,

When comparing to the rational choice, I did what I do in real life: added a safety margin. If anybody tells me there is a 10% chance for a project to succeed, I will always subtract a couple of percentage points for unforeseen circumstances. You treated it like a school test, looking for the "right" answer. To paraphrase Nicholas Nassim Taleb, if a coin falls head 100 times straight, the chances it will fall tails the 101 time are 50% only in academia - in the real life the coin is fixed.