Why is my overall score not that bad when almost all my impressions are of low quality?

If all of your impressions are either served outside of your campaign country, have hidden/generic referrers, or are not in-view, then all impressions will be of low quality.

However, you may have that the majority of the impressions are only violating one of the three metrics, so in the calculation of the overall quality score, they will be given a lesser weight.