Meat temperature differences.

I am doing my second cook of boston butts. I ran into the same thing on the first cook. Would like to know if it is something Im doing wrong, Probably is. I place butts centered up on lower rack with small space between them. Right now the meat temperature on the one closest to the fan is running about 30 degrees higher than the other. Both butts are within ozs of being the same weight. I have the bottom variable damper pushed in five inches. Should I be setting the damper in a different position. I thought pushed in 5 made the heat distribute evenly. Any advise or suggestions would be appreciated.
Thanks
John
Thanks
John