minor nitpick, what you mean is bilinear filtering, which is what comes "first" within a mip-map level. Anisotropic filtering is what makes texturing at angles much sharper (especially streets in racing games...). edit: this is wrong ;) aniso requires the wider border
That's not like that. Everything is atlased into a giant single texture layout for a bunch of assets (you can check the individual files in your program files x86 folder), and then that's split up into small 128x128 square tiles. Because anisotropic filtering wouldn't work otherwise, each tile has a 4 pixel wide border, so…