Unexpected caster change readings

[doubleload]

I’ve read about other forum users efforts to achieve positive caster and I have successfully modified double A-arm front suspensions for increased positive caster, but this T-bird is a real challenge. In my experience , as the front wheels swing 20 degrees right and left of the straight ahead position the wheel angle changes equally in both directions. However, on my vehicle the drivers side wheel leans 2.5 degrees as the wheel is rotated to the left 20 degrees and only 1 degree when the wheel is rotated 20 degrees to the right. What causes this unequal change?