Why would 5mm retrolisthesis at l4/l5 be seen on 1 lumbar xray and not another months later, or another 2 years later? does retrolistesis heal?

Movement possible: Spondylolisthesis is when vertebrae move relative to each other. So it's possible to see it on one film &not another. Also 5mm is small amount &one radiologist may see it (or think she sees it) &other may not. Some minor findings are in eye of the beholder. Sounds like spine has been stable; should be reassuring. Be careful of focusing on spine too much. Can lead to more pain &unneeded procedures.