FBWA mode problems

I have an APM2.0.

If I set my plane 100% level on the bench then switch to FBWA mode the ailerons move to try and make the plane turn right a little.

I calibrated the plane level in Mission Planner a few times but nothing changes.

How can I stop this?

If I increase the P value for roll it only affects 1 aileron (the right). The left travel stays the same.

I have a flying wing so have it configured for elevons.