In UTM projection, distortion is small near central meridian, when you move away it increases. A and B are near central meridian in UTM Zone 8. When you convert them to UTM Zone 10, coordinates of A and B are calculated based on central meridian of UTM Zone 10.
After convertion, points are still in the same location on earth, but far from central meridian of Zone 10 in comparison with Zone 8's. Therefore, distortion of A-B distance is bigger. This is nature of UTM.
