Because the sugar hasn't soaked into the banana from the skin yet. I once had a too raw banana and the inside of my mouth went ooky, agh, I never did that again. Speckled bananas would spike us more I think. I'm gutted I forgot to measure after my banana, I really wanted to know how much it would spike me.If the bananas are yellow, then they send my levels high, if they are turning from green to yellowish I can normally get away with eating them.