Black sensors are only correct when the tank is brand new and unused. After that it's a crap shoot (sorry for the cheap pun). "Stuff" will glom onto the sensors and cause false readings. I think TP is mostly the culprit. I've noticed after a good cleaning (rinse, rinse, rinse...) it will show empty. After a couple of flushes, boom, 2/3 full. I'm guessing the splashing gets whatever is glommed onto the sensor wet and the tank reports full again.
There are sensors that claim not to foul but I don't think its worth the effort.
2013 F-350 CC SB 2WD 6.7PS
2013 Eagle Premier 351 RLTS
-SOLD- 2012 X23B
-SOLD- 2003 Ford Expedition 5.4, Bilstein shocks