An example (from a case I was involved as a witness in only last week). A jury must be 100% confident that the evidence presented confirms their beliefs. The judge will say "do not decide based on if you believe it happened but instead on the evidence presented". There a world where the jury can believe it happened (and deliberate for 4.5 hours) but still come out with not guilty because the physical evidence is not there. In historical sexual abuse cases (of which this was one) - the CPS (Criminal Prosecution Service here in the UK) will not take on the case if they did not think there was a good chance of winning and yet the only real evidence here was witness testimony and no forensics. It didn't matter that the jury obviously believed it happened - they did their job and realised that was only an opinion and not backed by evidence.