Since the air we breathe is about 60 percent nitrogen, the largest contributing factor is the amount of air passing over the meat. On my vault, it will be more pronounced cooking on the top rack (reverse flow smoke and air from top to bottom). If I cook on the bottom rack just above the water pan where the air leaves with the rising steam just above the meat, no ring. Middle racks don't get much of a ring either...best theory I have is the cooker is so efficient, it takes very little air to maintain the fire and the air after the top rack spreads out or dissipates where the meat doesn't get as much. Still gets heat, just not as much air flow over the meat.
I use a 10 cfm guru on the vault. The Yoder pellet has a 100 cfm fan that is always on. The smoke ring is remarkably more significant as a result in my scientific opinion. Same thing for offsets...they use more air.
Believe it or not. :blah: