I don't know, I mean the apprentices Braig, Dilan, Even, Aeleus, and Inenzo all followed Xehanort and conducted experiments. They casted off their flesh, yet they retained normal heartless while Xehanort became a superior body of darkness, why is that? I'm sure they all had similar interests in the darkness, they all used the power of the darkness, but they dissapeared and Xehanort became all powerful and all knowing. Sora, who had a strong heart, accidently released his heart, and it was reborn as darkness, but only a small heartless was created. Although Sora's heartless was weak, it retained some sense of self, perhaps Sora and Xehanort are not so different afterall.
After Sora's heart was restored to the light, his physical form was restored as well, with the help of Kairi. In chain of memories, Axel was very shocked about that fact.
But with Sora's return from the darkness granted him with some unique abilities. Something much similar to Riku's dark form. Nomura said that Sora's Anti-form follows a similar case with Riku's dark form. Perhaps Sora has more ties to the darkness than we think.
I believe the reason why Sora retained a small heartless, was because he could not control it, more or less he subconscienceously avoided the darkness. The apprentices retained normal heartless maybe because they themeselves lack the strength of heart to maintain such darkness at the level of Xehanort, after losing their bodies, perhaps that could be a reason. But Xehanort was special.
When ever a heartless is created, a nobody is created as well. The heart is reborn as darknesss while the body and soul that's left over is sent to the realm inbetween and is reborn as a nobody. But those of strong hearts get nobodies that resemble the original body, the appearnce may differ slightly. Usually the nobodies obtains all of the memories of the origina body, but that is usually. Roxas, Namine, Xemnas, and possibly even Axel were created in speical ways. I can't remember where, but it was mentioned that Axel returned to his original state.
