Center feed and right side exhaust will always cause more airflow on the right side and much better circulation. Along that line, heat would be drawn to the right causing that side to maintain a warmer temp.
I'm wondering if your ribs on the left were just not as tender because they were not cooked as much due to the lower temp. Did both racks pass the "bend test" or how did you determine they were ready to come off the pit?
Also remember, the meat plays a big part in the outcome. I just did 3 full rack of ribs from a single cryovac package this weekend. They all fit side by side on my pit (so received the same heat) and the smallest rack (thickness wise) actually took 45 min longer to cook than the thickest rack as tested by bend-test. They were all delicious but expect different cooking time and listen to the meat to tell you when to remove it as opposed to a stopwatch.