Not too sure, but I'll assume that they're going by volume then, rather than weight. When you really think about it, 2oz is a little vague, it's doesn't quite state weight (wt) or fluid volume (fl). NOTE: Just looked up fluid ounces, and this statement from wikipedia clears up a few points: http://en.wikipedia.org/wiki/Fluid_ounce In either system, the unit is often abbreviated as fl. oz., oz. fl., or fl oz. In the USA, it is common to refer to the unit simply as an "ounce", especially in cases where no confusion can occur with the unit of weight. So it's a cultural thing, I guess. "oz" in the US is used interchangeably between volume and weight, it just needs to be taken in context. With that in mind, I'd assume jars are categorized by fluid ounces rather than weight. It makes sense, since you can somewhat standardize how much volume goes into the jar, but not how heavy the substance will be. Hope this helps!