First off the main component that schools classify intelligence by is memory processing. In "gifted students" it takes 2 or less repetitions of information for the student to recall it. In average students it takes 7-12 repetitions. In below average students it takes 15+ repetitions.

Highly intelligent kids in school do a lot less work/effort than bright or average intellegent ones. They don't to put much effort into repetition to outperform everyone.

When lots of effort is required later on in life, the highly intelligent ones often fall behind.

In my opinion the graph is wrong on the top. Excellence is usually the higher than average intelligent people, not the top end intelligent ones.

Most graduate students are from bright/average students. They are the ones with the combination of intelligence and effort to succeed.

I think I know what you're saying. Hope you don't mind if I take a stab at it.

Knowing failure and overcoming challenges is a skill. A skill that often is underdeveloped by intelligent people in their youth because of being naturally ahead of their peers early on. Some intelligent people finally hit a challenge where they need this skill later on in an environment that is less forgiving (like in college) and absolutely flounder.