All over the world drinking habits differ depending on cultural history, legal regulations or price level.
If you order a drink in a bar in Sweden the bartender would ask you if you want 3, 4 or 6 cl of alcohol, because that is the way we have served and priced cocktails and drinks here by tradition, counting the total amount of alcohol in the glass.
Other countries have different standard serving sizes and that is why all recipes from ABSOLUT usually are in parts (fractions) to maintain the right taste. Learn the drink by parts to get the right taste and then transform it to the appropriate serving size for your occasion.
One example – the famous Black Russian is made out of 2 parts ABSOLUT VODKA and 1 part Kahlua poured over ice. In Sweden that would be 4 cl of ABSOLUT VODKA and 2 cl of Kahlua to make a 6 cl Black Russian.
In the US, where oz is the measurement, maybe the drink would be 1 oz ABSOLUT VODKA and 1/2 oz Kahlua. The total volume (and price) will be different but the taste will be the same since the proportions of the ingredients are the same. As you may know. 1 oz is approximately 2,8 cl.