high altitude smoking

I would have kept it at 225.

Boiling point lowers to just about 210 deg F at 9,600 feet, so doubt it was altitude. Sometimes I find some cuts are just drier than others, especially with Prime Ribs for example. One of the best I ever cooked myself was at high altitude in Flagstaff, Az. I attribute it to a lucky really good quality meat selection from the local grocer. I sure didn't do anything special or different.

I'm at 7200 ft. and my times seem to match that of others at lower altitudes.

Agree with mountainman. I lived at about the same altitude as him and never noticed much difference. I also agree with lonzinomaker about leaving it at 225. Maybe the brisket flat was just a particularly lean one.