Carbohydrate values in McCance and Widdowson (M&W) series of publications are expressed as monosaccharide equivalents.

These values can exceed 100g per 100g of food because on hydrolysis 100g of a disaccharide, such as sucrose, gives 105g monosaccharide (glucose + fructose).

Thus white sugar appears to contain 105g carbohydrate (expressed as monosaccharide) per 100g sugar.
For conversion between carbohydrate weights and monosaccharide equivalents, the values shown in Table 1 below (adapted from M&W introduction) should be used.

In trying to explain this to students (depending on the age) you could explain this using chemistry and molecular weights:

Sucrose + water → glucose + fructose

C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ + C₆H₁₂O₆

342g + 18g → 180g +180g

So in this example you can see 342g of the disaccharide sucrose gives 360g monosaccharides.