Correct me if I am wrong, but the question 'have you packed the bag yourselves?' is not a mandatory DfT requirement, instead it is used by some check in agents to lead onto the mandatory questions which are:
1. Could anyone have interfered with your bags/baggage since you packed it?
2. Have you been given anything to take on board your flight?
In essence the first question would cover most scenarios, e.g. leaving out of sight, not having packed it yourself etc etc.