If we're talking water, you'd be correct. However, not all liquids are created equal. 1000mg = 1g; this is a unit of mass, or "how much" stuff there is. On the other hand, 1 cc (cubic centimeter) is equal to 1 ml (milliliter); this is a measure of volume, or the amount of "space" taken up by the stuff. Water is by definition 1g/ml, but other liquids can have different densities. One gram of a denser liquid (for example, glycerine) would occupy less than 1ml of volume. By contrast, one gram of a less dense liquid, like ethanol, would occupy more volume. Since different substances have different densities, something that's 10% of a solution, by volume, may not necessarily be that same 10% when measured by weight.