The origins of the 1/3 stop meme may have come from the Forward of the standard. It's in the 1979 version and may be in the earlier versions. "(3) For any specific developer, the sensitometric speed rating method of this standard includes a 1/3 stop underexposure safety factor." This refers to where the average shadow exposure falls in relation to the fractional gradient point and is explained in the paper Safety Factors in Camera Exposure.