Perhaps overconfidence is an american trait (coming from "think positive"), because over here I thought and heard we were generally undermining ourselves.
I talked about it to a person looking for someone for another job within the company. She agreed we normally have a higher potential than we think. While at the army, everyone took part to a jogging and I was passed by a guy who was going faster , followed him (we talked while jogging about our hidden potentials) and I passed all my comrades, ended first of my section (and had all the showers available).
Maybe there's a lot more culture involved in this perception.

Raoul