This is true in nearly any case but frozen fruit.
Vitamin C diminishes in fruit and vegetables from the moment they are picked. However, freezing slows down this process so much that it seems it stops it at all. Fruit and vegetables to freeze are harvested at their best and frozen within hours from harvest time, so those frozen raspberries in the frozen food section may contain more vitamin C than the raspberries on the fresh fruit section which needed at least 48 hours, if not longer, to get there.
Frozen fruit may have more nutrients