Ravel,
Good to see the birds came out well! I'm following your thought process, and I think Gregg is right on. First, the air, at the top of the smoker, is more stable. Secondly, the bottom shelf is closer to the heat source (radiant heat). However, that large hunk of meat, below the top hunk of meat, not only acts as a heat sink, it acts as a shield! Hot air rises straight up, and the meat above rests in a "dead air" space. Yes, the air flows around the bottom meat, but not directly. I'm not a physicist, but my experience has always shown that the top meat is done in more time than the bottom meat.
Don't stress about it, or over-think it. Just remember that the lower shelf gets done first.
As for your Auber dropping to 218: Sounds to me like it was trying to recover from all the door opening. In the future, put the probe in the center of the breast meat, from the front, and just keep the door closed until it hits 165.
