What an odd way to divide things. Without more info about the relationships between everyone, it's hard to say but grandpa either liked some more than others or trusted some more than others, or felt someone needed the money more than the other.
I've always felt while people should will their estate to whom they wish, that if they really loved their family, they'd do things equally. Either divide the estate equally to their children and not worry about the grand kids, or give everyone the same percentage. Even then it can get sticky.
My friend's grandmother died, with a very detailed will. Each of her three children was to receive $X and each of her grandchildren to receive $Y. But, of the three kids, one had no children, one had only one child and my friends family had the most, three children. My friend's father's siblings thought that this was unfair, that my friends father's family "got" more of the estate just because he had more kids. Never mind that all the grandkids (my friend and her two sisters) we're all adults and got the money in their own right, they still felt that my friends father got more money than they did (he didn't, he got what they did. Not his fault they didn't have kids, or as many kids).
Rather than fight it in the courts and have the lawyers take it all, my friend's father settled, they had to pay the estate back a large sum so it could be re-distriubted to his siblings. Needless to say, they all don't talk much any more.
I'm lucky, my two brother's and I all know what's in my father's will, and it's very simple. We each get 1/3 of the estate. That's it. No worries about someone having more grandkids, or someone being liked more than another, or someone "needing" the money more than another. We each get an equal slice. If one of us blows through it or otherwise makes poor choices, oh well.
