Why does there seem to be such a big difference in case mortality between the USA and Europe?

At the time of writing (11 July 2020) the USA have almost 3.2 million confirmed cases of coronavirus infections, while Europe has slightly over 1.8 million with the USA passing Europe around 10 May 2020 (two months ago) at around 133 thousand cases. But if you look at deaths then Europe has almost 185 thousand, while the USA have (only) 134 thousand. On 10 May 2020, Europe’s death total stood at almost 151 thousand and the USA have not yet caught up. Taken together these numbers seem to imply a much lower case mortality for the USA than for Europe, for which I have not been able to come up with an explanation (other than that the numbers are bad).

Why does there seem to be such a big difference in case mortality between the USA and Europe?

EDIT: since 13 September 2020, both death totals have reached the level of 194 thousand with 6.5 million confirmed cases in the USA and 3 million in Europe.