Many engines have the crank bored off center fromt he cylinder, that forces the piston skirt to one side and the same side at all times, if turned counter clockwise it will tilt to the opposite side and have more or less compression.

To the TT .91 owners. The compression of this engine will go up and down till it is completely broken in. I don't mean the time it takes to run well, but the time it takes to get the best compression possible, probabally 5 or more gallons of fuel. Till then the ring will rotate, differance in how much oil is on it, how thin the oil in the fuel you last used, and other factors will cause the compression to vary.