It appears that the confusion has been pretty well worked out, so this is not timely enough to help in this case but might in new situations.
The key to remember is that an ounce is NOT the same as an ounce.
Sound nonsensical? Welcome to the the English System!
The "ounce" where 1 cup is equal to 8 "ounces" is really not called an "ounce". It is called a "fluid ounce" and is a measure of volume. You can call it anything you like. You can call it a Fred.
The "ounce" where 16 "ounces" is one pound is properly called an "ounce" but it is a measure of mass (not force - that's yet another "ounce" that is properly called "ounce-force") but the fact that the word "ounce" is used as part of both names is due purely to the historical evolution of the system of weights and measures. Again, you could call it anything you like. You could call it a Sue.
A Fred and a Sue are clearly not the same measures. There is seldom this type of confusion with the metric system because a cup would be equal to so many liters and there is no automatic tendency to equate liters to kilograms. The confusion comes about solely because of the use of the same word to mean two different things in the English System.
A simple example makes it very clear. If you pour water into a 1 cup measure you have 8 FLUID ounces of water and if you weigh it on a scale, it will show that you have a bit more than 8 oz - it would show about 8.33 oz because, contrary to the old saying, a pint is NOT a pound the world around. But if you pour mercury into that same 1 cup measure you will still have 8 FLUID ounces of mercury but the scale will show that you have 109 oz of mercury - or more than 6.5 pounds!
Look at the bright side - ounces and ounces are easy compared to pounds and pounds.
