Risk involves some form of estimation, as Flyvberg has shown, the best estimates are based on past, factual reference data, but for most projects that’s hard to obtain and so project managers are left to rely on their estimation skills. There’s an exercise on estimation here that I think everyone should do at least once. Most people are over confident in many areas.
Do you think you are a better driver than average? 93% of people put themselves in that category (of course it should be 50%). The same is true of estimation, and it’s an area that’s key to risk management, the temptation is to be excessively precise in one’s estimates, and this level of false precision understates true risk.