The piston is pushed to one side of the cylinder when being pushed up by the crankshaft pin. When this happens, the skirt of the piston helps seal the compression.

In your clockwise direction the piston skirt seals the compression lost by the gap in the ring. In the counter clockwise direction , the opposite side piston skirt is help the non gap side of the ring seal the compression. This leaves the gap side of the ring with no sealing help and you notice less compression.