JohnS is correct. There is a difference between fluid ounces and ounces [weight]. Here's an example
Fresh water weighs approximately 8.33 pounds (108.29 ounces) per gallon (128 fluid ounces). If you divide 108.29 by 128 you find that fresh water weighs 0.84 ounces/fluid ounce.
Sea water weighs approximately 8.6 pounds (137.6 ounces) per gallon (128 ounces). Using the same equation you discover that sea water weighs 1.075 ounces/fluid ounce.
Apply that to freon which has a specific gravity of 1.21 in liquid form. This means it weighs 1.21 times more than water (which has a specific gravity of 1) within the same volume. So if we multiply 0.84 (the weight of fresh water/fluid ounce) by 1.21 you discover that 1 fluid ounce of freon weighs approximately 1 ounce.