ok apparently this has been talked about a few times on the forum... the below link is but one such thread. apparently this is intentional and the best response I have read is that it has to do with reducing drone. by crimping one pipe and not the other it changes the stereo effect associated with how drone sound is formed when coming out of two identical cross-sectional pipes. I highly doubt it has anything to do with clearance issues or manufacturing or assembly line stuff.
Have a look: https://www.allfordmustangs.com/forum...-pipe-wtf.html
2013 V6 Mustang Automatic with the following mods: Roush Hood scoop, 3M Clear braw paint protector, MRT rear window louvers, 3M scotchprint carbon fiber roof wing decklid and scoop, strut tower brace, FRPP axleback exhaust.