The reason water with additives is a little different , is that once you change the composition of water it becomes more alkaline or acidic.
what this means is that it will either be more permeable in the bowel and cross into the body , or it will encourage other elements in the body to cross into the bowel and be excreted with it.
For example if you drink lemonade instead of water when you are sick and already dehydrated, it will draw more fluid out of your body and be excreted and you can become more dehydrated.
thats why sports drinks have a certain makeup to discourage this and aid in rehydration.
if any one has had a colonoscopy and drank the prep drink to clear out your bowel, this is the principle it works on drawing fluid into the bowel.
I'm not a hundred percent sure about diet cordial as it doesnt have sugar I add it to soda water to change the flavour, when I'm sick of water.
not sure if this helps at all