Why do people achieve less than they are capable of?

(from Quora: click here)

They don't.  We to tend over-estimate our capabilities. The truth is, we achieve exactly what we are capable of. Abilities on their own don't mean much. They are defined by our achievements.

How do you know a car can go from 0 to 100 km/hr in 5 seconds? You have to drive it and find out. If it fails to achieve that, even after trying it under different conditions - it was not capable of doing that. Simple.