This is exactly the case. A big problem is that both the following can be true: a) most task estimates are greater than the time required b) most project est

This is exactly the case. A big problem is that both the following can be true: a) most task estimates are greater than the time required b) most project estimates are lower than the time required even though the project estimates are just the sum of task estimates. Partly this is because there are unexpected tasks, but it's also because errors in estimates are a fat-tailed distribution with a lower bound (a task can't take negative time). So the median error can be negative even though the mean error is positive! But, because tasks happen more often than projects, the availability heuristic means that managers place more weight on a) than b) even though b) is what is actually important for the business.