Just because a product is deemed paraben-free doesn’t mean it couldn’t contain some sort of preservative to extend its longevity and prohibit the growth of harmful bacteria.
While there are a number of different methods and ingredients that can be used for a more natural form of preservation, several companies choose to use natural antioxidizing ingredients like grapefruit and grapeseed extracts, and vitamins A, C and E. Organic phyto-extracts and essential oils help keep a formula together and effective.
Additionally, some brands may even use less water, or none at all, because more water used in a formulation means more susceptibility to spoilage.