What "reality?" The "huge contrast" could be with Kyon's reality of his own low grades, or with the predictions of his theory regarding Haruhi's inability to stay focused in class. It just cannot be that the reality of her top notch results are in contrast with...the reality of her top notch results.The catch was this: I was fine with not paying attention in class, but it was almost impossible to not think of anything else, and even if I did, would life be happy that way? I don’t think Haruhi would want to lead that kind of boring, mundane life. As I started to gradually doubt her words, I suddenly recalled that her results were top notch, a huge contrast with reality.
I suspect the huge contrast is between the reality of Haruhi's grades and the attention deficit disorder diagnosis Kyon puts on Haruhi's behavior. If so, the fix could be to substitute "her erratic behavior" for "reality."