Well, it's partly because Xemnas was lying to us all along.

[That statement comes with a smile, maybe the first Terra has seen on his face since they met. But it's wry, not happy, a kind of 'Can you believe I believed that?' kind of smile.]

He didn't want his heart back like most of us did. He just wanted power. Plus, I know now that my original heart was somewhere I couldn't reach. But it turns out none of that mattered.

[Terra's been listening really closely, which is encouraging, so maybe he can explain more than he thought he could before his backup arrives. Roxas lets his Keyblades dissolve into nothing and rests one hand on the center of his chest.]

This is going to sound crazy, but...it grew back. I have a heart again. Which probably makes me the weirdest Nobody ever.