I know I'm going to hate myself for saying this, but my sensors are almost always correct. And yes they are the standard bolt through the side of the tank sensors. Well once in a while the black sensor sometimes sticks at 1/3 but I've never had a false 2/3 or F reading that a fill and drain didn't take care of. I could probably do the same on the 1/3 sensor, but that one just doesn't bother me enough to want to waste the water to clean it.
To answer the original post though, yes the stock sensors are fairly inaccurate.