When I use to clip my temp probe in-between racks I found that the temperature reading could be 20°F or more different from the sensor that was located on the bottom rack. The sensor in-between racks would always register lower (much lower at the beginning of the cook). The difference in temperature would depend on the load, and how close the food was located to the sensor. Evaporation from the meat cools the area around the surface of the meat, and you tend to get a lower temperature reading. So if you are going to wall mount it, where you have it seems to be the best spot. If you moved it to below the bottom rack, it would be too close to the element. Maybe monitor your cooks more closely the next few times. The bottom rack may cook much faster, and you may have to rotate your racks.
When I did this test while the cabinet was empty, the temperatures between the two sensors would be within 5 degrees of each other.