How much difference are you talking about? Have you checked the ride height at the front side to side?
There are so many factors that can affect the ride height at each corner of the car.
The first things to check though is that the floor is absolutely level and tyre pressures are even side to side.
As you have a composite spring you can be fairly sure that that is okay. Supposedly they won’t ever sag (instead just snap completely!!) Your bolts are set the same so the problem lies elsewhere.
Try disconnecting one end of the sway bars front and back (if there is one at the back) These could be bent or not set correctly, and will affect the ride height if wrong. Check there’s nothing amiss with the front springs too, the front affects the back.
Really, a cars ride height is taken as a chassis to the ground measurement at each corner. Then if it’s a competition car, corner weights come into the equation too. This might all be correct but the car still looks wrong when viewing the wheel arch gaps or wheel arch to ground measurements. So it’s down to how the body and panels are aligned and sitting on the chassis and if any areas have ever been repaired etc.. Adjustment to these things is difficult or impractical, so rightly or wrongly……most people end up adjusting those rear bolts if there’s nothing else obviously wrong on the car! I would’t want to see too much difference in the bolt lengths though - you’re hiding a problem elsewhere.
