The Focus Distance Upper and Focus Distance Lower tags are in the Canon-proprietary "maker notes" and aren't part of standard EXIF, so documentation is scarce. However, it appears that these (together) represent the distance at which the lens's focus is set. That is, it's somewhere between the two bounds.

Why Canon does it this way rather than providing a single value (perhaps with second uncertainty or error value) is a mystery only Canon could resolve, and they don't seem to have publicly. Here's a forum post with some investigation: Re: Distance in EXIF.

Note also that Depth of Field is an exiftool composite tag. The information there is not set directly by the camera — instead, Exiftool generates this from other data, including whatever hint it gets as to focus distance. From the docs, it looks like it uses the Upper and Lower values, but also other info, which may explain the disparity you note.

In any case these values are approximate and should be taken as hints, rather than as gospel. Their original purpose is probably to hint at subject distance to aid the camera in making TTL flash power computations. They're not meant to be scientific measurements, or even photographic scene information (like GPS location tags or something).