Calibre has a minimum required size of 775x420. Snap will not override minimum window width requirements. If you have a 1366x768 screen, half of that is 683 pixels, which is obviously less than 775 pixels. So calibre will snap to its minimum of 775, resulting in some overlaps.
On my 1920x1080p laptop, calibre snaps just fine to half-screen because 960 > 775.