I have noticed that difference in sound too, and I also feel like there is a difference in hardness. The stuff made in a sealed retort seems to have the same density, but it stays in much larger pieces, and does not crumble as readily. The stuff done in an open burn almost seems puffed - could there be a pressure component to it?
Your retort is looking good - how many runs have you done with it? I am curious if you use anything beyond a steel band to seal your lid, and if you have any trouble with smoke leaking out around the edges? How much surface area do you have in the holes in the chimney for the wood gas to exit? I feel like my retorts have always leaked irritating smoke around the rim- but maybe I just didnt have enough exit holes and was building up too much pressure.