[chrismcb]

There is quite a bit of water in soda. While there is a lot of sugar, it is about 10% which is a bit less than 100. Specifying per 100g (or some other size) makes it quite easy to compare two different products. Of course you also have to take into consideration how much of the product you will consume

[bagacrap]

The percentage being expressed is (calories from X)/(total calories). For soda and sugar, that comes to 1.