As of this moment (januari 2014) XBMC doesn't understand the HEVC codec which is being used in this particular mkv container. The HEVC codec (which is also known by the proposed - but unofficial - name h.265) is a new format which has not been finalized. The only files that you will see in the wild are made by people that wanted to test out the new proposed encoder for this format.
Internally XBMC uses (http://www.ffmpeg.org/%7Cffmpeg) to decode videostreams. Quite recently the (http://www.ffmpeg.org/%7Cffmpeg) maintainers included support for the proposed version of the HEVC decoder (ffmpeg version 2.1) This version of ffmpeg is not in use in the most recent builds of XBMC and since we are not allowing any more new features to go in to v13 'Gotham' development (we are in feature freeze in preperation to release) this feature will not make it into the first release of that version.
Of course we are planning to upgrade the ffmpeg we use to a newer version. Because ffmpeg is such a core component it touches a lot of parts of the code, and might very well cause regressions. (For instance a first test to upgrade ffmpeg to the latest versions resulted in a severe memory leak which caused 16GB of memory usage within 20 minutes.) Obviously these issues need to be resolved before you will see a version with ffmpeg 2.1 and there are developers working on it, but the main focus at this point in time is on releasing XBMC 13.0 'Gotham'
You can be sure that by the time the H.265 encoder is finalised and people start using it widely there will be support for it in XBMC, but at this moment we are not there yet. Therefore the best suggestion is to avoid using this encoder for the moment if you intend to play your creations with XBMC. Currently there are no hardware accelerated options to play HEVC content, so you would need a pretty beefy CPU to decode a highly compressed HEVC movie!