I'm looking at panniers right now and wondering about weight distribution, specifically; why, if the optimum weight distribution is 60 front/40 rear, are all these panniers sets rigged with the large bags in the rear and often with a rack bag to boot? Is it just a question of practicality or is the idea to load the really dense compact items up front (stove, shoes, etc.) and the bulkier, lighter items (sleeping bag, clothes, etc.) toward the rear?

I'm having a hard time figuring out how one would achieve the optimum weight distribution with these things.