What the health score means
The score starts at 100 and loses a weighted penalty for each open finding. The weight is how much money the fault stands in front of, which is not the same as how broken it is: a missing product image costs more than a long meta description even though neither is an error.
Each of the four categories is capped, so a shop with 1,800 products missing alt text cannot lose its whole score to one fault and hide everything else. The penalty also grows with the square root of the affected pages rather than in proportion, because the second instance of a fault tells you less than the first.
A muted finding does not count. Muting is you saying it is intentional.