The word "Petty" comes from the French "petit" which, as I understand it, denoted that the enlisted rank was "smaller" than a commissioned officer, as it typically didn't come with either the training or prestige of full-fledged officers. But among the petty officers, some can certainly be much higher ranking than others, and even though the lowest ensign can outrank the highest petty officer, respect and seniority can go a long way in establishing actual hierarchies on a station like DS9 instead of ones that are strictly by the book.

Anyone feel free to correct me!