Many factors could be causing the differences here. Age and physical condition are the first to consider along with how many levels were fused at one time. Results seem better when two levels are fused compared to one. The results of three levels versus one level are unknown.
Type of fusion can make a difference, too. Was the operation done from the back of the spine, from the side, or from the front? Were plates and screws used? What was the source of the bone graft? Each of these has a little different healing time.
Sometimes early dependence on a brace or support actually makes rehab last longer. Exercise to regain motion and strength are important early on.