Metals , when cast , are not completely homogeneous. So differences in density occur which translate to different weights...
The " off center" i was talking about referred to the distance from the center than any potential weight anomoly occured... the total weight difference could be the same between two flywheels... but if it is right near the middle on one as compared to being near the starter ring gear it makes a huge differnce in the vibration which it causes...
The concept is like ' torque arm' though that may not be the exact word for it. The assymetrical weight times the distance from center times the rpm is a close description of the total damage potential.