You've got behavioural screening and object screening. On this one occasion the object screening failed, but it didn't matter because the behavioural screening works so well. At the end of the day, if someone with no murderous attempt accidentally leaves his gun in his hand luggage and brings it onto the plane, no harm is done (as long as no-one else gets hold of it, which again isn't a problem if everyone on the plane has been successfully behaviourally screened).