OpenGL coordinates are for the bottom-left of the pixel. So drawing a line from 0,0 to 1,1 fills exactly one pixel (in screen coordinates).
Put another way, image a piece of graph paper, with 0,0 at the bottom-left. If you draw a line to 1,1 you will get a diagonal line in the first grid square - it will not extend into the square at 1,1 as below.
The diagonal represents a line from 0,0 to 1,1 - filling exactly 1 pixel at coordinate 0,0