File -> Export Bitmap -> Selection

There is always a 2-pixel margin at the four sides of the selected object in the exported png file. Why? How can I remove it?

The image is to be used as a header banner for a webpage, 1000 x 100 pixels. When I create the rectangle of this size, somehow the size will later on become 1004 x 104, andI figured out that those extra spaces are the margin described above.

Thank you.