Because your genetics are also from your grandparents! You have 50% of your genes from Mum and 50% from Dad. Each of them got 50% from their Mum and 50% from their Dad. So, because of this the 50% you get from each of your parents is at the same time 25% from each of their parents! Does that make sense?
Sometimes traits seems to skip generations too, that’s all linked to how certain characteristics are expressed because of how genes are passed on.