Interesting test. I need to try this a few times. (Plate, no plate, changing exhaust position...)
Noticed it again yesterday on a load of ABT's. One side was get done faster. I would spin the rack and rotate the lid so the exhaust would be in a different part. (Sorry - no after pic on this load - and as fast as they went, I needed to make about 3 times as much!)
I have a shorter but wider basket - about 17" wide and 5" of height.
You can just see in the second pic that when I start it in the middle, it doesn't always work itself out evenly in the first few hours or the burn (or if I have something smoking that is a heavy dripper)
I thought it was my 2" exhaust that is to the side (flat lid). Need to get to know the drum better.