The Vaccinum Myrtillus is closely related to the American blueberry, Vaccinium corymbosum. The reason the dried is 600% more antioxidants than fresh bilberries is simply because the moisture has been removed and the compounds hold up well if they are stored properly.
ORAC Source
Research Support, Non-U.S. Gov't: Carlsen MH, Halvorsen BL, Holte K, et al. Nutrition Journal NIH 2010