Complex sentence: The 13-tooth sprocket is non-reversible and has a slight outward offset, changing the chain-line slightly, this because the smaller sprocket makes the chain run closer to the axle, and the chain would rub the hub shell if the sprocket were not offset.
Split into simpler sentences: The 13-tooth cog is non-reversible and has a slight outward offset, changing the chain-line slightly. This because the smaller cog makes the chain run closer to the axle, and the chain would rub the hub shell if the cog was not offset.