I think its because "they" have formulated a nanoparticulate form of zinc oxide which doesn't leave that white residue but may penetrate the skin. The basic variety that you can buy in the pharmacy is likely to be the old fashioned type, which just sits on the skin acts as a barrier. If its in a product that you use, then you need to ask the manufacturer, though I would suspect that any company with a conscience (weleda, neals yard remedies etc) would be well aware of the controversy and avoiding their use.