This is not a bug. The 1024 x 768 built-in preset is intended to re-size the browser's viewport, not the window, to show a web page like it would display on a tablet. (The viewport is the area where a page is shown, excluding all the window borders and toolbars; this is meant for developers to test the responsiveness of the pages they are creating because, as the description of the extension says, this is primarily a developer's extension).
So, you asked for a viewport of 1024, your display scaling is set to 125% and your left and right window borders are 10 pixels each, so you have 1024 * 1.25 + 10 (left) + 10 (right) = 1300 pixels. Same thing goes for the height, except the address bar and the titlebar at the top of the window are 105 pixels so you have 768 * 1.25 + 105 (top) + 10 (bottom) = 1075.
Anyway, if you need the actual window, including borders & stuff to be 1024 x 768, you need to open the extension's settings, go to "presets" and add a new one or edit the existing one, making sure that the "Resize target" is set to "Window".
Customer support service by UserEcho