I'm not surprised in the least. Your fire basket is smaller than the drum. With that design the outside edge is always going to be a bit cooler than the center.
That is why many UDS people have a ~8-10" deep fry candy thermometer mounted on the side of their drum. You get a better idea of what the temps are like in the center that way.
You could add a third rack with a platesetter (ala egg). Pizza pan, terracota saucer etc. That will even out the temps a bit.
It was one of the first things I noticed on the poppa kit. Short thermo. Look on the bright side, you can brag that your drum cooks up everything faster.