@qeeM My guess, snap to grid is on and a 1 pixel grid. Using free select down the grid and back to the origin. That gives a selection that passes through most of the pixels.
geeM - Gimp is is a raster editor and works with pixels. The minimum unit is a pixel. However there is a sub-pixel effect when you come to fill or paint the selection. There is an anti-aliasing effect depending on 'how-much-of-the pixel' with partial transparency depending on what you do, paint / fill / ...
Give some information on what you intend to do with the selection and you will get better advice.
geeM - Gimp is is a raster editor and works with pixels. The minimum unit is a pixel. However there is a sub-pixel effect when you come to fill or paint the selection. There is an anti-aliasing effect depending on 'how-much-of-the pixel' with partial transparency depending on what you do, paint / fill / ...
Give some information on what you intend to do with the selection and you will get better advice.