You're right, and I believe "How the mind works" by Pinker goes over this and the evidence for it. It makes perfect sense for grandparents to pour more resources into grandkids who are 100% certain to be yours. There are plenty of other similar phenomena that are determined by the estimated genetic overlap, like domestic abuse/violence is almost entirely between genetically unrelated people, i.e. between spouses, step parents and their step children etc. The more related the less violence.