To understand this you have to understand what is being measured:
The NHS has lost its prestigious ranking as the best health system in a study of 11 rich countries by an influential US thinktank.
The UK has fallen from first to fourth in the Commonwealth Fund’s latest analysis of the performance of the healthcare systems in the nations it studied.
Also, why it is being measured. The Commonwealth Fund desires that the American health care system be replaced with something much more like the NHS. Thus their measurement system is created to make something like the NHS look good:
Analysis of 71 performance measures across five domains — access to care, care process, administrative efficiency, equity, and health care outcomes
The actual health care bit is only that last. Any and every system that has equality of access will do well by such measures whatever the excellence – or not – at treating people.
Access to Care. The access to care domain encompasses two subdomains: affordability and timeliness. The five measures of affordability include patient reports of avoiding medical care or dental care because of cost, having high out-of-pocket expenses, facing insurance shortfalls, or having problems paying medical bills.
Equity. The equity domain compares performance for higher- and lower-income individuals within each country, using 11 selected survey measures from the care process and access to care domains. The analysis stratifies the surveyed populations based on reported income (above-average vs. below-average, relative to the country’s median income) and calculates a percentage-point difference in performance between the two groups.
The entire measurement system is biased towards equality rather than actual health care.