I don't get the impression that Draco is especially brilliant (for a real eleven year old, he would be, but Eliezer's characters don't act eleven in general,) but rather that he's especially well trained. He might be a one-in-a-hundred intellect, but he's had an education that not one muggle in millions gets.

Blaise is clever, but likewise learned from an exceptionally duplicitous mother, and had Dumbledore passing him notes.