I look at it differently.
An AD is due to a hardware problem. Something broke and you didn't know, shot goes off...you, in the grand scheme, had no control over that shot not going off. It was going to discharge do to a hardware issue.
If you knew of the hardware issue, and it happens, the see below...
A ND, is because you just weren't paying attention to the rules. Something stupid happened that you could have avoided by just being more diligent, and a shot went off.
An AD is due to a hardware problem. Something broke and you didn't know, shot goes off...you, in the grand scheme, had no control over that shot not going off. It was going to discharge do to a hardware issue.
If you knew of the hardware issue, and it happens, the see below...
A ND, is because you just weren't paying attention to the rules. Something stupid happened that you could have avoided by just being more diligent, and a shot went off.