I looked it up; the rule is for the shoulder line to be three times the head's width, i.e. you can fit three heads across the line.
I've also looked into anthropometric databases. For an adult male the average head width is 6.1" whereas the shoulder line is 18.3". This is a shoulder line to head width ratio of 3.
For a small person, the head width can be no smaller than 5.6" and the shoulder line no smaller than 15.8". This is a ratio of 2.82.
For a large person, the head width can be no larger than 6.7" and the shoulder line no larger than 20.6". This is a ratio of 3.07.
So, if we want our head to look normal, the shoulder line should be 3 times our head's width, a ratio of 3.
If we want our head to look small, we increase the ratio towards 3.07.
If we want our head to look large, we reduce the ratio towards 2.82.
Of course, reducing the ratio may mean that the shoulder line is smaller than your real size, so the suit won't fit you. I believe in these cases the slope of the shoulders has a visual effect. Even if the ratio is perfect, i.e. 3, slopping shoulders can make the shoulder line look smaller and your head larger. Here, the suit must compensate for the slope.
To answer your question, we don't know but 20 is normal.
I think having "broad shoulders" has to do with your frame size and height. For example I'm 6'3" and my shoulder width is approximately 22 inches. I'm muscular but I'm not a body builder type. I have a large frame. Most people consider me to have broad shoulders.
If you want a scientific explanation I found this on firefighters link which states their average shoulder width to be 20.8 inches with a standard deviation of 1.4 inches. So in relation to firefighters you are average while I'm one standard deviation above average statistically. Firefighters are usually in shape and lift weights so they may alter the bell curve a little bit. =P