If you've tasted straight distilled water made without the use of any pre or post carbon filtration, it might have a steamy taste or off taste. Not using a post filter in the past with distillation might have produced an off taste in distilled water, due to these VOC'S. The carbon post filter is most important because it acts as a polishing filter to clean up any volatile gases, which can produce an off-taste, that may have escaped during the boiling process. Second, another reason for distilled water having this label for 'flat taste' is someone accustomed to drinking chlorinated or well water high in iron content for a long time. When they are suddenly introduced to distilled water for the first time, it is a shock, noticeable to their taste buds. People raised on high iron in their well water for instance, would be used to its 'sweet' taste, but when given distilled water might say it tastes 'flat' since it is iron free.
Taste is the number one reason that consumers buy bottled water.
Many consumers are led to believe that you need minerals in water to give it its taste. It is actually oxygen that gives water its taste. Water shouldn't have a taste or a metallic after bite. A third reason for the 'flat taste' theory is buying distilled water in the store. Distilled water, as virtually all bottled water, is stored in cheap plastic containers.