@Selen
only 18%. It means that out of 100 visitors, 82 will see a blank spot, no ad.
This is true, but it is not the case for everyone, and you need to understand the subtleties of how the ad-balancer works and what this means for you.
In your adsense account there is report called revenue profile it's the last or second to last on the list of reports. It shows a plot of distribution (cumulative) of revenue by the number of impressions. Along the Y-axis you have revenue in RPM and along the X-axis number of impressions (or % of total impressions). So for say $0.10 RPM impressions you should have like 95% of all impressions (left hand side of the plot), then at $0.50, you'll have 30% of impressions and so on, your top earning impression will be on the far right. The typical shape of such a plot is a curve starting low at the left and climbing to the right.
What the ad-balancer does is truncates this graph at some point along the x-axis. If you set your revenue to 100% really it is 99.5%, it goes along the x-axis to find the number of impressions that match 99.5% of revenue, say that $0.12RPM. It then removes any impressions that pay less than 0.12$ RPM. The next day it will re-estimate this price floor and re-adjust, it may shift the price to $0.13 because now there is more demand.
When the 100% (or 99.5%) revenue mark is a 82% of impressions what this signals is that your curve is no longer a curve, but a pretty close to being a flat line. Under these conditions, the ad-balancer is essentially ineffective, because it will be extremely difficult for it to predict an accurate cut-off price. In other words it will have trouble determining if the ad should or should not be blocked because nearly every ad lies on the threshold.
The other thing it tells you is that there is very little demand for your inventory. Basically, nearly all your ads are low value and potentially spammy ads. You may want to look into your content to find a way to attract new advertisers.