Because they believed what tehy were told and it was false.
"Essentially, when they saw they could not convince the majority in favor of voting guilty, they conceded their votes, believing that they did not have the option to stick to their guns and prevent a unanimous verdict," Stillinger wrote in the motion.
Gourley said he thought the foreman was relating something he heard directly from the judge and when he found no mention on hung juries in the court's printed instructions, "I had no reason to doubt the foreman," he wrote.
After the trial, Gourley told the media that he felt pressured by other jurors who wanted to resume their normal lives after more than two weeks of trial. He also said he thought 10 years in prison was a grossly inappropriate punishment for the agents.
"Had we had the option of a hung jury, I truly believe the outcome may have been different," he said.
The third juror, Woods, wrote, "I don't remember exactly what it was that made me change my vote to guilty on these charges, but I know I was very influenced by my belief, based on the other juror's statement, that we could not have a hung jury. I think I might not have changed my vote to guilty if I had known that was an option."
http://www.dailybulletin.com/news/ci_4508579
I have never seen a jury verdict overturned becase a juror was too stupid to stick to their convictions.