An Emacs frame has an internal border and an external border. The internal border is an extra strip of the background color around the text portion of the frame. Emacs itself draws the internal border. The external border is added by the window manager outside the frame; depending on the window manager you use, it may contain various boxes you can click on to move or iconify the window.
When you specify the size of the frame, that does not count the borders. The frame's position is measured from the outside edge of the external border.
Use the ‘-ib n’ option to specify an internal border n pixels wide. The default is 1. Use ‘-bw n’ to specify the width of the external border (though the window manager may not pay attention to what you specify). The default width of the external border is 2.