Professional translators Lucais Sewell and Christiane Focking write in The Translation Post about the difficulties in arriving at "appropriate and honest translations when mediating between the divergent conventions of the American and German grading systems."
They, too, note the "clear discrepancy at the top of the scale" and observe how due to grade inflation in the US, anything less than straight-As is "less than spectacular," whereas Germans tend to be "extremely reserved in their praise," which results in the greater difficulty in earning the top mark. Interestingly, Sewell and Focking notice this reserve on the linguistic level as well--the difficult-to-achieve German "1" is translated as sehr gut ("very good"), whereas the American "A" is designated as "excellent."
The authors correctly comment that the very respectable German report card with all 2s comes across as mediocre in the US. The World Education Services scale below correctly translates that German report card as a straight-A report card.
http://translationpost.com/index.php/tag/german-grades/