As someone who has served on a federal grand jury this is true (the rarity of a false bill), but is most likely due to two factors I saw over and over again:
1) The government generally doesn't bring borderline or weak cases in front of a grand jury. They have limited time and budget and don't want to pursue a case they don't feel is a sure thing (or, more accurately, a sure deal with the defendant as most cases do not go to trial).
2) Grand jury decisions need not be unanimous, only simple majority. I voted against indictment in a couple of cases I felt were weak, but they were indicted none-the-less. Further the burden of proof is not beyond a reasonable doubt but rather a preponderance of evidence[1], which is a lower bar.
1) The government generally doesn't bring borderline or weak cases in front of a grand jury. They have limited time and budget and don't want to pursue a case they don't feel is a sure thing (or, more accurately, a sure deal with the defendant as most cases do not go to trial).
2) Grand jury decisions need not be unanimous, only simple majority. I voted against indictment in a couple of cases I felt were weak, but they were indicted none-the-less. Further the burden of proof is not beyond a reasonable doubt but rather a preponderance of evidence[1], which is a lower bar.
[1] http://en.wikipedia.org/wiki/Legal_Burden_of_Proof#Preponder...