Right, should have added that to the list of not-quite-consistent explanations. Also, slight typo, black level is 16 not 10. And to this I'll add another specu

Right, should have added that to the list of not-quite-consistent explanations. Also, slight typo, black level is 16 not 10. And to this I'll add another speculative answer, similar to others that have been advanced, but I'm not sure it's ever been explicitly expressed: it means that a piecewise linear approximation in either direction can be done with good accuracy and not a huge number of segments.