First, you can't really tell from this data whether an American actually produces more than a German in the same job. The data covers all the jobs in the country, and if the US happens to have a higher fraction of employees in highly paid jobs like investment bankers, lawyers, doctors, etc., it would raise the average.

Also, do the data for the US account for all the work that employees do after hours, like answering e-mails and phone calls? Since the kind of employees that do this are usually exempt from receiving overtime pay, these extra hours wouldn't be recorded anywhere.