Why are bananas rated "low" on the Glycemic Index?

Bananas are a fascinating fruit in terms of their carbohydrate and sugar content. Even though bananas are a fruit that tastes quite sweet when ripe - containing 14-15 grams of total sugar - bananas receive a rating of low in their glycemic index ...

from The World's Healthiest Foods http://ift.tt/2b9NlnY