Why does white bread have a higher gi (glycemic index) than whole grain bread?

Definition: The Glycemic Index (GI) is a relative ranking of carbohydrate in foods according to how they affect blood glucose levels. Carbohydrates with a low GI value (55 or less) are more slowly digested, absorbed and metabolized and cause a lower and slower rise in blood glucose and, therefore insulin levels. Relative to your question, which bread has more fiber? Ref: http://www.glycemicindex.com/