How is the glycemic index computed from a blood sugar curve?
The AUC of identical dosages of sugar depends on digestion speed because of the rate at which sugar is removed from the blood stream. If sugar exponentially decayed in a first order rate, then the AOC would be the same, but it's save to assume some 0 order mechanisms contributing. E.g. alcohol is mostly metabolized in a 0 order reaction rate: A fixed amount per hour.
Edit: If you take up sugar slowly, it might never reach high concentrations in the blood stream and lead to lower AUC then if taken up fast. Each molecule has a longer lifetime if taken up faster.