The US rate of 8.1% (https://www.cdc.gov/nchs/products/databriefs/db67.htm) is for all deaths. The Japanese rate of 11.7% (https://www.japantimes.co.jp/opinion/2016/02/27/editorials/t...) is for "unusual deaths". (Yeah, I wish I could find better figures, but all I get are abstracts for papers about the reasons for Japan's low rate of autopsies; the papers are behind paywalls.)