Mike thanks, your screen shows the 0,0 really at the top of the document. For me that is slightly higher, and can not move downer with pulling the cross downer.
To demonstrate the difference I was able to snap a pink circle to the grid, and a yellow rectangle to the top of the document, so from very slight point of view but they have different coordinates

grid.xarClick image for larger version.  Name: grid.jpg  Views: 122  Size: 23.2 KB  ID: 88644