To be more specific, the reason for that single line is because some legal systems (like in the US) have different "classes" of offense. That line is the differentiation for which class the crime is. The lesser could be a "summary offense" or "misdemeanor" and the higher a "felony"

I, of course, was not suggesting that you didn't know the difference in seriousness. I am stating that the difference in seriousness is the reason for the line, because the underlying legal system generally differentiates crimes this way.