I would theorize that the reason one packet is 2 servings is because one packet is to be added to a 0.5L bottle of water. That is about 16 ounces of water (16.9) and 1 cup ie 8 ounces of water is a "serving" of water. Also, if they made one serving packets you'd have to add 2 packets to a bottle of water, which is less convenient and produces more trash, yadayadayada.
Of course, One packet is still two sweet for me in a half liter, so I often use partial packets anyway, when I'm using the water flavorings
I'm on the "Eat-as-much-as-possible-while-still-losing-weight" diet. It's working.