Usually it means that the bananas are just not ripe yet. However, I have purchased bananas in a store that I waited to ripen and turn yellow, only to find that they were already ripe and soft, and ready to eat. I think there are different "species" of bananas that are shipped in from various countries. You may just have a bunch of them. If they feel soft, peel one, and see if it looks ripe. Otherwise, they were just picked too soon off the tree and shipped to the store. Try placing them in a bowl by a window. The sun will help to ripen them faster (I hope!) That, or you could hire a monkey to test them out for you!