The reason you seem to develop asymmetrical abs MAY be because the pelvis is not aligned properly. This is really common, and easy to fix.
The human spine is often under asymmetrical loads, and the way we walk, sit etc. can cause some mild rotation of the pelvis - so that you will find yourself listing slightly to one side during crunches/back extensions etc.
Go to a chiropractor/osteopath - if your mechanics are functioning correctly, you'll get much more out of you training.