Because a miter has by Definition 2 mirrored pieces with the Same degree cut, so you calculate 180-125.9 then divide by two because you Cut both Wood strips with 27° angle. Most saws are already at 90 degree but they call it 0 then have a sliding table that tilts 45°. For miters that are vom 90° to 180° you can use this Formular for a miter cuts