Romola Wills say whatever the testator wants them to say.
Yes, in many families that are united and cohesive, if an adult child dies then often their potential in heritance will be left to their children, but there are plenty of wills where that doesn't happen for all sorts of reasons, and none. In this case, where the son and grandchildren had ignored their parent/grandparent, not visiting him, even when he was ill. The grandparent decided to leave his estate to the children he had that cared for him and he saw regularly. Others would have done differently.
My grandfather, who had a large family, left his whole estate, essentially, just the house he lived in, to the unmarried daughter, who had lived at home and juggleed working and care of her parents.. All the other children knew what their father's will said, and everyone was in total agreement that this was the right thing to do.