The share of voice in the PPC Performance Tracking module refers to the number of ads that are visible in comparison to the total number of ads present on the results page, multiplied by 100.
The share of voice differs from the Coverage Rate since Share of Voice takes into account the pages where the site is not present (which is not the case for the coverage rate).
Out of the 10 times that your site was checked, it was not present on the results page 8 times. Your coverage rate will be 0% for these 8 checks.
The Share of Voice however, is the average of the Coverage Rates of the 10 times it's been checked, regardless of whether or not your site was present. The Share of Voice is therefore 8%.