range hood specs?

I tried to replace my parents' range hood with one they purchased from Lowes. Turns out the vertical exhaust opening center point is 5" from the back of the unit while the existing was 6". Old one was a Kenmore and new one is Broan.

So the hole in the cabinet floor obviously doesn't match. And it isn't oversized enough to get the exhaust pipe in the collar. I know I can always take a jigsaw and open the hole in the cabinet up... but that would leave a big gap on one side. Also I am wondering why this dimension isn't standard and being that it isn't why can't I find the spec for that measurement on any range hood box or product info web page? Are all Broans 6" and Kenmores 5"?