The coordinate information is shown in a World file (tfw) and the Geographic Imager panel are different because World files use the center of the pixel to derive coordinate information while the Geographic Imager panel uses: 1) the top left corner coordinates of the pixel placed on the very top left corner of the image, and 2) the bottom right corner coordinate of the pixel placed at the very bottom right corner of the image. The second and third values in the World file are zero because there is no rotation or skew.
When saving to GeoTIFF, it is possible to choose a different reference method: Pixel-As-Area or Pixel-As-Point. Pixel-As-Area uses the upper left corner of a pixel as registration. Pixel-As-Point uses the center of the pixel as registration. A Geographic Imager preference allows you to keep the existing pixel reference point of the current image or override by choosing Pixel-As-Area or Pixel-As-Point.