This doesn't explain much 

After reading the article I still found myself wondering how grades are calculated. The table would suggest a 9% grade is merely 5°, but I was on a 9% road today and it must have been at least 30°. The linked page  offers a much clearer answer, that the grade is simply 100*(rise/run). Perhaps this is not a common standard but it is certainly used in Northern Alberta and BC. -)

Though, I still don't understand why percentages are used at all. -/