that's because there are two different sizes of DP's, full length and shorty. The full length replace both stock DP and midpipe, connecting directly to the catback. THe short AM DP only replaces the stock Dp and leaves the stock midpipe and midpipe cat. understand?
if you get a shorty there's no need to worry about catback fitment, you already spent enough time on that. If you go Full length (which you should to eliminate bottlenecks in your exhaust IMO), then you need to figure out if it's flat flange of donut flange (for both DP and catback).