it has everything to do with it. if the murder rate remains constant and a group doubles in size, then on average twice as many people in that group will get murdered.
they didn't expect it to be, they said if it remains constant.
You seem to be the one making the assumption -- that it wouldn't remain constant -- otherwise why would you have made that comment?
If I am making the wrong inference here then I put it down to the strong and yet imprecise language of "it has everything to do with it".