Table sugar is composed of sucrose which is broken down into half fructose and half glucose and AGAVE can contain up tp 90% fructose.  That increase in fructose can mean that agave nectar does not result in the same dangerous spikes in blood glucose that other sweeteners do

Such high fructose content is not typical of all agave products, "Depending on how the syrup is processed, it may or may not contain fructose