80/20 Rule AKA Pareto Analysis

OK for those of you that don't know this rule, I'm sure you've heard of it. The premise of it is by doing 20% of work you can generate 80% of the advantage of doing the entire job. Article is at http://www.mindtools.com/pages/article/newTED_01.htm
if your wondering. The article goes on to say that you need to write out a list of changes you would like to see done. If the list is long then, group related changes and score each item depending on the relevancy to the situation and/or problem. Then take on to change that which has the highest score that you assigned because it will give you the biggest benefit when and if you solve it.
Now what I don't get is why it 20/80? Why can't it be like 25/75 or 30/70 or something else? Or even a fraction or decimal? Or maybe it might mean that, hypothetical, if a 10% amount of work was done then 90% of the entire objective perhaps? mmmmm maybe I think.
But it is useful to find out that major problem that you might be facing and if you solve, it will help you out 80% (more or less).