As you see, editing 3D models is sometimes not as simple as it seems.

The reason for that strange cutting off half the pixel is the way how 3D engines map textures to surfaces. They always render from the center of the first to the center of the last pixel. Thus the render area is one pixel smaller than the actual texture size. For compensating this, the texture scale is a little enlarged.

If the engine would render from the start of the first to the end of the last pixel, as one would normally assume, the surface would get visible seams because the last pixel mixes with the first. So for displaying mapping you must always make a compromise. The 2D window shows the correct mapping, even if it may look strange to you.