I believe there was a DNS test in that case, after which the police decided not to proceed - which would seem to imply that in that case the false alibi was to cover him for a case in which he was not actually guilty. If so, that might have made her more likely to have gone along with the alibi this time.

However I still think this is a peripheral issue. Huntley did in fact have a criminal record even aside from this - and it never showed up in a way that stopped him, getting the job. How could that happen, and how often does it happen?