I've owned many rigs, and they all had the same sensors, which are basically short metal rods that probe into the tank. The sensors have never been reliable measurement devices. The black tank is the first to go. Any sort of trash (and there's plenty trash in that tank) that attaches to the probes will cause it to misread.
I just live with it. As the salesman at the RV place told me, they use the same sensors in some of the big RVs (that costs over $100G) and it's the same problem. Oh, well. When the toilet looks full, we dump. We rarely dry camp, so it's not a problem for us.