What does "scratch below the surface" mean?

In Outlier: The Story of Success Chapter 2 Section 4, there is a sentence saying

Is the ten-thousand-hour rule a general rule of success? If we scratch below the surface of every great achiever, do we always find the equivalence of the Michigan Computer Center or the hockey all-star team - some sort of special opportunity for practice?

I look it up in LDOCE5 and Merriam-Webster but only find what scratch the surface means. Googling this expression seems only examples rather than a good explanation.

Solution 1:

Scratching the surface is related to scratching below the surface. When you scratch the surface of something in the non-literal sense, you're just beginning to learn about something and only know a few superficial details. When you scratch below the surface, you've learned more information about a subject. At that point, you have moved beyond a simple understanding of the subject and are delving into the depths of it, to use another idiom.

Both scratching below the surface and delving into the depths are similar in their idiomatic meaning in that they both imply that you are learning more about a subject.

Solution 2:

I respectfully disagree with Shaun that "scratching below the surface" has a standard idiomatic meaning. "Scratch the surface" is a cliché meaning to not go too deeply into something. It's usually used in the phrase "barely scratched the surface." I think the writer here is trying to extend and freshen the cliché by turning it on its head: "scratch below the surface" would mean to definitely go deeply into something.