問一個語法問題，謝謝老師們~~~

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

 請問為什麽我的句子被改成這樣。 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~~~