When you think you know some area, you become less opened to new knowledge in this area, so in result it appears you'll not learn new, and will know less afterwards.
The more you're in position you have not enough knowledge, the more attention you'll be paying to otherwise unnoticed details, and when having actual knowledge, multiple times increases quality of final work.
Socrates's sentence was just generalization of this phenomena.
Try this idea on your self and you will notice personal effectivity boost. (I did).