Volume percentage (V/V): Solutions that are formulated from two liquids are often expressed as volume percent with respect to the solute. The volume percent is the volume of a liquid in 100 mL of solution. The label on a bottle of ordinary rubbing alcohol reads “isopropyl alcohol, 70% by volume”. Such a solution could be made by mixing 70 mL of alcohol with water to make a total volume of 100 mL, but we cannot use 30 mL of water, because the two volumes are not necessarily additive:
What exactly does "not neccessarily additive" mean?