I agree that it seems very arbitrary. I got a G/VG, G/G, and VG/VG and I got one. (Computer Science - other, bioinformatics) I'm guessing that the other category of bioinformatics helped and my broader impacts is really good although my academics aren't the strongest (mediocre GPA). I had heard from past people who got it that the broader impacts was almost more important than the research so I really played on that and how I had an international internship, senior project, eagle scout, etc. To anyone applying again I would basically just say try and concentrate a lot on the broader impacts and try and reiterate almost word for word each of the things they list in the instructions as to what they look for in the broader impacts. I did just that and made sure I covered everything.