Question

How are Zekrom and Black Kyruem on the same game and why is Reshiram and White Kyruem on the same game?

Answer 1

Gamefreak Wanted it like that. I mean, would Getting black kyurem by combining Kyurem and Reshiram Have any sense? NO.

Answer 2

In Black Version You Got Reshiram And In White You Got Zekrom So They Wanted Each To Be Given To Their Roots. Leaked Footage For Black 2 Shows N's Zekrom Fight Kyurem and Fuses With Zekrom And Became Black Kyurem But In White 2 N's Reshiram Fights Kyurem and Fuses And Becomes White Kyurem.