When? If you were in the 1800s, it might have been an easy standard to follow.
A fathom is 6 feet the depth the old law quoted above. A fathom was used for measurement for hundreds of years and may have just been an easy measurement to specify. Just like today when I see articles on size people tend to say 10 feet, 20 feet, 10 meters, 20 meters. I know that an article where I see 33 feet the person told the writer 10 meters. Where is this going? I was demonstrating how we end up with weird measurements, that used to be a nice round figure that made since. I think the fathom may be tied into the early burials and the English law put it in feet instead of the original fathom, which when asked how deep do we burry him, the seaman sid a fathom deep. Wag, but an interesting possibility.