The Worse-Than-Average Effect: When You’re Better Than You Think

sometimes the competent don't know when they're competent.
This is the worse-than-average effect
This means that when you're good at something, you tend to assume that other people are good at it as well.
So, when you're faced with a difficult task that you are good at, you underestimate your own ability.
The moral of the story is simple: sometimes we do ourselves down, especially when faced with a difficult task or when we have special skills. Under these circumstances we are better than we know.

http://www.spring.org.uk/2012/06/the-worse-than-average-effect-when-youre-better-than-you-think.php