The US has some of the best healthcare providers in the world. They also have some very bad ones. But gains are harder to achieve than losses in healthcare, so when you average in the bottom of the barrel you get some bad numbers even if both the median and the best available provider is above the international average.

Basically, healthcare quality in places that are both very rural and very poor (e.g. West Virginia and Mississippi) can be awful and it brings down the average even though the system provides better outcomes for most people in most other parts of the US.