While it may seem off that inserting an extra length of wire improves things I suspect a plausible explanation may be that the extender improves the quality of the connection between wall wart and socket. Wall warts are often heavy and pull down on the wall socket, in turn they're likely to pick up vibration further affecting the consistency of connection. Using an extender can allow you to position the wart/transformer away from the wall socket in an ideal location and hence deliver the improvements noted.
Interestingly I had a similar experience using a short extender where I needed some more length on a wart powered device and found far from making it sound worse it sounded a tad better (and I used generic non grounded extension cords) -- I'll look into making up some better short connectors so I can explore this further
Interestingly I had a similar experience using a short extender where I needed some more length on a wart powered device and found far from making it sound worse it sounded a tad better (and I used generic non grounded extension cords) -- I'll look into making up some better short connectors so I can explore this further