Spondylolisthesis or slipping of one vertebra over the other are usually taken care by the surgery if it fails to give relief with conservative means. It is not necessary to get them 100% reduced, but the most important part is to relieve the neural elements of all the pressure, which is caused either by the bony vertebrate or the disc and prepare the vertebrae for fusion.
In case of lumbar spine, spondylolisthesis need not to be reduced fully 100%, especially if the patient has a high grade listhesis, it is not desirable to reduce it completely. An important part of surgery is to clean the pressure of the spinal and nerve roots and prepare a healthy environment for bones to fuse.