The trouble with U.S. measurements is that there are many different meanings of the word "ounce" -- it could be a fluid ounce (volume), a troy ounce (weight), or an avoirdupois (also weight), all of which mean different things.
Most of the time, when we talk about an "ounce" we refer to the unit that is exactly 1/16 of a pound, or 28.349523 grams.
It's so confusing. I bet a lot of people, even in the U.S., would want to switch to measuring with grams, kilograms, and liters for this very reason.
--Metal Substance