This might be my headcanon, but I’m pretty sure I heard that Nomura said this himself, but…
Remember how DiZ (aka Ansem the Wise) responded to Roxas telling him how much he hated him?
Roxas: I hate you so much.
DiZ: You should share some of that hatred with Sora. He’s far too nice for his own good.
That’s exactly what Roxas did, intentionally or not. Because Roxas hadn’t peacefully merged with Sora (that’s what he does near the end of the game like Kairi does with Naminé), he effectively put Sora’s personality into a blender with his own. Sora still acts fairly goofy (especially in Disney-verses, such as his childlike joy over getting to meet Santa Claus); but as far as the Organization is concerned, Sora’s own sense of justice is magnified by Roxas’ righteous fury. That’s exactly why the Organization members often refer to Sora as Roxas (and sometimes even flip-flop between the two); they can sense their traitorous comrade and the influence he has on Sora’s actions.