问一个语法问题，谢谢老师们~~~

1）He said this was not an issue, the real issue being that Mary's mother has too many children.

 请问为什麽我的句子被改成这样。 我的是：

  He said this was not an issue, the real issue is that Mary's mother has too many children.

ps: Pls don't change mary's mother's children to mary's siblings; that is not the content, I have changed the real content, just for the question on grammer about "being" and "is" .

Thank you very much~~~