Lol, there is no meaning at all. Actually the word is used by who ever it creators might be only to meant ulitmate statistics. The Z (Being last in English alphabest) symbolizes the ultimate and stats for statistics. Other than that there is no special meaning attached to it.
Zstats.com is different from zstats used by minyurl.org
While creating minyurl.org which is a short url site, the underlying short url script for minyurl.org was, unaware of the existence of Zstats.com, named as zstats
After that they learnt about the existence of Zstats.com but have not yet changed the script of their short url site from zstats.