On all model engines...
Depending at what direction the connecting rod is pushing on the piston when it is headed up in the combustion chamber, (left or right), one or the other of the piston skirts will be against the cylinder wall. With the ring gap to the side and that skirt away from the wall, there will be less compression.

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On some four strokes....
The less compression in the clockwise direction has to do with the cam timing. On a saito, it's almost unnoticable. But some other brands it is very pronouced. One of the valves opens up sooner in the clockwise direction and releases the compression.

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With a four stroke throttle completly closed, and after going trough one compression stroke, most will have no compression on the second compression stroke.

Enjoy,
Jim