The share of voice corresponds to the number of ads where the site is visible compared to the total number of ads present multiplied by 100. The share of voice differs from the coverage rate in the sense that this index takes into account verifications where the site is not present (which is not the case for the coverage rate).
Of these 10 checks, your site is absent 8 times. Your coverage rate is 0% for these 8 checks.
The share of voice is the average of the coverage rates of the 10 checks, regardless of whether or not your site is present. It is 8% here