Both programs are showing you the same info. If you add the
-n option to your ExifTool command, it will return an orientation of 8, the same as XnView.
Read further down on that page you linked to the "EXIF Orientation Value" chart. When it says "left bottom", that doesn't mean that the left bottom corner is supposed to be the top left corner of the image, which would be a 90 deg CW rotation. An orientation of 8/Left Bottom means that the first row of pixels is the left side of the image and the first column of pixels is the bottom of the image. They do it this way because the orientation tag isn't only there to tell when images need to be rotated. The orientation tag also allows for mirror images.
If you notice on that chart, the values of 2, 4, 5, and 7 indicate that the image is "flipped" or "mirrored" (the term ExifTool uses).
These orientations are rarely used, at least I've never seen them used. Though I can think of one situation to use them. If you were scanning slides or film negatives and saving them as jpgs, for example, and put them in wrong, you could use these orientation tags to correct the images without having to deal with recompression.