The reason this works is that pixels are square, the same thing happens with normal maps. if your UV edge falls diagonally across a number of pixels, it has to take every pixel it falls across and interpolate it to find the closest match. If your UV edge is straight, the software has a clean edge to fall across and there is no interpolation.
So, anywhere uv edges are not perfectly horizontal or vertical you're going to get this effect, with ANY map, just the nature of the beast. You just dont notice it int he center of islands because it has more to interpolate rather than the edges.
You WILL notice it with normal maps int he center of islands though, with any normal edge that cuts across pixels.
Ah... the challenges of game art