Generally speaking, (if we discount other factors such as age, pre-existing health conditions, and trip duration) the difference in premium can be attributed to the cost of medical treatment in different countries.
If we look at the cost of a prescription to treat an ear infection for example, a week’s worth of treatment can cost around 6€ in Greece. But in the USA, you’re more likely looking at about $50. At almost eight times the cost for antibiotics, you’ll be claiming for a higher amount, so on this basis your travel insurance is going to cost more.
The same rule of thumb applies even within Europe - Belgium charges less than Spain, Switzerland is more expensive than France - and not all countries in Europe accept the GHIC card.