I figure it's the same sort of thing as yards vs yardage. When you're talking about yards you're talking about some specific amount, when you're talking yardage

I figure it's the same sort of thing as yards vs yardage. When you're talking about yards you're talking about some specific amount, when you're talking yardage you's talking about some unspecified amount that gets measured in yards usually. When talking clothing you're talking about an abstract concept, when you're talking clothes you're generally talking about some fairly specific clothes. There's a lot of grey area here, e.g. a shop can either sell clothes or clothing, either works to my ear.