Can something have more sugar per 100g than the percentage of sugar that's in it?
Looking up Shreddies, I found this site. It lists, in the ingredients 


  Whole Grain Wheat (96%), Sugar, Invert Sugar Syrup, Barley Malt Extract, Salt, Molasses, Vitamins and Minerals (Niacin, Iron, Pantothenic Acid, Folic Acid, Vitamin B6, Riboflavin)


There is no percentage for the sugar in the ingredients list. 

And the nutritional information says 


  Carbohydrate  70g 
  of which sugars   13g


If that's where your confusion comes from, then it is simply that you didn't realize the different meanings of the word "sugar". 

Chemically, sugars are a class of molecules with a roughly similar structure, most of which taste similar. For a cook or food technologist, "sugar" is any ingredient that constist of one or many of these molecules and can be used to sweeten food. And finally, in everyday language, "sugar" without any further qualifications is exactly one of these products, namely white crystal sugar, that consists of the molecule sucrose only. 

In the ingredients list, the second ingredient uses this third meaning of the word sugar - they have put less white table sugar than whole grain into the cereal (making the white sugar amount at most 4%). The nutrition label uses the first meaning - it sums together all chemical sugars in the cereal. And it is normal for even unprocessed whole grain to contain some of those - this being a cereal, and a malted one at that, it has more of them. So, part of your "whole grain" ingredient is made up of (chemically) sugars, as well as the "sugar" ingredient, the "invert sugar syrup" ingredient, possibly the "barley malt extract" (pure malt is quite a bit of sugar), and the "molasses" ingredient. Together, the weight of chemical sugars is 13% of the cereal.