Doepfer

A basic question-

If you plug stereo cables into the A100 sockets, the "ring" part is
grounded, and the "sleeve" part is unused.  In stereo signals the
"ring" carries the right channel signal.  But if you plug in
headphones to a mono A100 socket outputting and audio signal, you will
hear the same signal in both ears.  Why is the right channel not
silent?  It seems the potential difference between ring and sleeve
would be between ground and ground, which should be zero.  Can someone
explain how this works to me?

Thank you,
Monroe