Quality Score is presented as a report card. In practice it’s closer to a credit score that Google itself partially controls the inputs to — and chasing the number directly can lead you to make worse decisions.
The optimization trap
Accounts that obsess over Quality Score tend to over-tighten ad groups and over-match keywords to landing pages, which can shrink reach faster than it improves efficiency.
What actually moves cost
Expected CTR and ad relevance move the number, but real-world conversion rate is what protects your CPA long term — and that doesn’t always correlate with a higher Quality Score.