In normal circumstances, it makes sense that grand juries would always indict - because prosecutors would not bother to bring a case before a grand jury if they

In normal circumstances, it makes sense that grand juries would always indict - because prosecutors would not bother to bring a case before a grand jury if they didn't think they would get an indictment. All you need to bring up charges is probable cause, which is a pretty low standard, and it should be pretty obvious to a prosecutor if they have it. So the cases are all pre-filtered just because the system is in place. Even if the grand jury system approved 100% of indictments, it doesn't mean that it isn't preventing bad indictments from getting through. The same reasoning holds true for high rates of approval for warrants and 0% of airline luggage going through screening contaning bombs. Trials are different because they have an extremely high standard - beyond a reasonable doubt. A prosecutor can be completely convinced in their own mind that someone is guilty of a crime and they have no choice but to bring them up on trial if they think they even have a chance of meeting the high standard, even though they may fail.