I remember reading about other exhaust that did the same thing.
Z -tec (spelling?), not really sure on the name, but members reported the same thing, one side had more flow than the other side.
It may be true that the muffler "favors" one side of the exit since its flows easier there.
Also yes I have bought many things from ebay myself (and get same comments from people here ) and they mostly turn out nice. But the price usually dictates the quality in the long run.
Like others have said to me, thanks for buying and testing that muffler out. Now we will have a member with 1st hand experience.