We overestimate how well we knew our grandparents. We have a picture based on memory we can attach a lot of things, even true things to, but we often didn't know those things or understand them very well at the time. Much of the shared time was when we were quite young and/or they were quite old. Our young selves have much they do not really understand, and our old selves are not always our most representative.
Few of us spend much time with even one grandparent after we are adults ourselves, for obvious reasons. One of my grandfathers lived until I was thirty, but I didn't see him more than once or twice a year over his last decade. Most of what I know of him is what my own father told me. That wouldn't have meant much if I didn't have some relationship with Grampa myself, plus some concrete memories of his house and the things he owned. But it still remains that on the day he died, I couldn't have told you much about him that I learned on my own.
I could tell you more now. When we become adults and have jobs, bills, wives, children, cars, and so forth, we can fit the pieces that we have into something more solid. Yet it's a retrospective exercise. I was very close to one grandmother, who lived nearby and whose house I went to for lunch every school day from 2nd-6th grade. I lived with her when my mother went into the hospital. Nanna died while I was in college - I probably know four times as much about her now, not least because the things I learned later - that her marriage was unhappy, that she lost her first child a few months after she was born, that she had to go to work after finishing 8th grade - were not the things we tended to share with children.