Also check to see if you a re taking your daily calcium and that magnesium products are taken two hours or later. That would be many vitamins, calcium, or antacids that contain magnesium. That would change your dosing.
Since you are still in a good range, the real question is are you feeling ok? Mine was adjusted four years after surgery. My endocrinologist says he sees this frequently enough to know that it is ok. If very ill it will change as well. If you have a major inflammatory disease it can change then too. That is why we check every six months. Good luck. Oh, and be sure you are on the same brand and not changing between the brand and the generic. That also makes a difference.