It's true that some patients have more motion than others after a total disc replacement (TDR). Doctors and researchers aren't sure why this happens. It's widely known that different people have different motion. In fact, different spinal levels within the same person have different amounts of motion, too. Maybe these two factors make a difference.
Based on studies of other joints and previous spine studies, it appears that patients with more motion before surgery have more motion and a better result after spinal surgery. Some research seems to suggest different TDR implants have different results, too. A slightly different design may make a difference in outcomes.
It's also been shown that early movement without bracing results in greater spinal motion. Doctors want to get to whatever is causing the difference so they can give patients the best results possible. More research is needed to sort this all out.