What is the difference between single tread and double tread slipper socks?

Single tread has the non-slip tread under the foot.

Double tread has tread on the bottom of the foot and on top of the foot.

Some patients find that the socks become twisted on the foot as they walk, especially if they are finding it difficult to lift their feet off the floor. When the sock twists, the double tread socks still provide the non-slip surface to grip to the floor and prevent falls.