Hi,
Its all hypetethical, but differentence in header lenght, can size, stinger diameter or lenght. Any leaks or restrictions in exhust. Try this, pull one plug and turn the engine over with ignition off of course. Then replace the plug and pull the other plug and turn it over. This will allow you to get a feel for the compression. If you can't tell the difference between the cylinders they should be within 10% of each other.

Dave