I think a little bit of air might manage to leak in through the exhaust port .

It's quite common for a well run in ringed engine to have better compression when turned backwards but I'd say there's something wrong if it has no compression when turned the normal way.

The usual explanation for the difference is much like Charley said. The ring(s) wear in to give a seal from the side thrust loads caused by combustion pressures. Turning the engine backwards simulates the same direction of loading (but now only with compression pressure) so the rings are sealing the best they can.