Sexual offences are notoriously difficult to get convictions on. Only 2% of alleged offence go to court and of them only 2% result in guilty verdicts. The odds are with the rapist every time. This is why the CPS only go to court with the strongest cases.
Huntley had had a lot of allegations made against him but hadn't been to court and found guilty of any of them.
Police checks only say if there have been any convictions; if there are none then the check comes back clean.