They are confused because 2 separate units share the same name: an ounce by weight is 28.35 grams and is measured on a scale, while a fluid ounce is 29.57 milliliters and is measured with a measuring cup. In water, the 2 happen to be very close, which is exactly why the confusion grows. With ingredients whose density differs from water, such as flour, honey, and oil, they diverge noticeably.
The most subtle trap in foreign recipes is that 2 separate units share the same abbreviation. An ounce by weight is a unit of mass equal to 28.35 grams; it is measured on a scale. A fluid ounce is a unit of volume equal to 29.57 milliliters in US recipes; it is measured with a measuring cup. In a recipe, the 2 units are often abbreviated as “oz,” and the ingredient is the only clue to which one is meant. One measures mass, the other volume, and the 2 cannot be converted directly into each other. When translated into Turkish, the 2 are often reduced to a single word, and the distinction disappears entirely.
There is a reason the confusion has lasted so long: in water, the 2 units produce almost the same result. 8 fluid ounces of water is about 237 milliliters, and because water’s density is close to one, it weighs about 237 grams too. The same amount calculated in ounces by weight comes to 227 grams. A 10-gram difference is not noticeable in a glass of water. So the most common place to make the mistake gives nothing away, and the habit takes hold. Because water does not expose the error, it can persist for years.
The difference appears and grows with ingredients whose density differs from water. The same volume of honey is noticeably heavier than water: using the measurement equivalents here, one 200 ml measure of water weighs 200 grams, while the same measure of honey weighs 280 grams. Olive oil is the opposite and weighs less than water in the same measure. If a recipe calls for 8 ounces of honey and those are read as 8 fluid ounces, weighing it will produce a difference approaching 100 grams, and the dessert will not have the right consistency.
There is a practical way to tell them apart. If an ingredient flows at room temperature and the recipe calls for it in a measuring cup, it is a fluid ounce; if the recipe calls for it on a scale, it is an ounce by weight. Recipes often do not spell this out, but they leave a clue: a fluid ounce is abbreviated “fl oz”, and those 2 letters settle the question. If the abbreviation is only “oz” and the ingredient is a solid such as flour, sugar, cheese, or meat, assume it means an ounce by weight; that assumption is almost always right. Checking the list of units at the beginning of the recipe also settles the matter at a glance.
This distinction is useful when reading packaging too. The “oz” value on a foreign product label means different things depending on whether the contents are solid or liquid, and it needs to be converted to the same type of unit before comparing it with a recipe. The only way to eliminate the confusion entirely is to change the system, not the units: convert solids to g and liquids to ml, then rewrite the recipe in metric from the start. That prevents errors and makes the recipe unambiguous the next time you read it.