Any bottle with a varietal label (i.e., Duckhorn Cabernet Sauvignon, Rombauer Chardonnay) is only required to contain 75% juice from that particular grape varietal. Up to 25% of any specifically labelled wine may contain juices from other varietals used to blend and balance the final product.
Take that nugget to your next wine tasting party! More on the way. xoxoSusan