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 transformation, 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.