You don't have to like the choice. Probably Oda won't indulge much on why Xebec had another child. Anyway:
1) clearly because for BB there was basically no susprise, It's was the time, while for Buggy It's Indeed a plot twist;
2) Buggy, like It or not, Is among the most important characters of OP;
3) I don't know how Oda can be more transparent tbh:
4) if that wasn't enough, Shanks got his secret world relevant lineage too.
You think Buggy, Shanks' "other half", will remain some random dude with no origins lol? And in this manga too, in relatives piece lol?
Yeah, we all see people trying to look dumb, even though...why lol?
Oda doesn't care about bullshit or last moment changes. See G5.