I like the objectivity kept in this article to come up with the formula and how you drew those conclusions. spot on.

For the topic of why we can't meet estimates, there are many subjective reasons that a developer is unable to stay away from. Those reasons could be - during estimation one wants to show they can do better and makes lower estimate; or they are fearful of performance and give lower estimate; or they are fearful of not able to complete and they give higher estimate; programming is mind-game and one cannot predict the mood.

For a manager, important is to keep a tab and help when someone is stuck. For employee, raise a hand when they are stuck and get rid of the fear.