My wild wild guess is that Shenyang aircraft has a beam that separates 2 large IWB thus the dark groove that was observed, I don't know the true purpose of that design, may be it is there to strengthen the internal structure to withdstand the impact of aircraft carrier landings. This aircraft has a wide belly, and these 2 IWB are not small or narrow as we might had visualised.
Again another wild wild guess. After the demostrator was bult and flew, they build 2 prototypes, one for airforce and one for aircraft carrier. Thsi is a very wild guess but not impossible, especially China always give unexpected surprises.
Also, I always expect that 6th gen has duel flight mode, manned and unmanned (as an UCAV), on a same aircraft.