This thread probably belongs in Technical Talk section, but UVs extending past the border is often used for tileable textures. The Coordinates just continue tiling outside of 0-1 space, it is also why you can offset overlapping UVs for baking purposes and still have them work.
I moved this into Tech Talk I've never experienced that issue, is this something that comes up at crazy scales? Laying out your UVs passed 0-1 is very important for environment art, that's how you tile textures. At one point I was making complete zones inside of 3d studio max, I would select all the terrain that was grass,…