You're actually taking the quote out of context. Getting a C in an art class is different from getting a C in macroeconomics. "Everything" doesn't refer to, say, athletic ability, I'm sure. There's some limitation to the set of abilities involved, centered around mental abilities, and art skills might be outside that limit, in the quotee's mind.
I mean, it's really easy to imagine a "good student" branching out, taking an art class, and getting a C, but it's hard to imagine a "good student" branching out, trying macroeconomics, and struggling.
I mean, it's really easy to imagine a "good student" branching out, taking an art class, and getting a C, but it's hard to imagine a "good student" branching out, trying macroeconomics, and struggling.