But aren't both "5" and "6" failing grades in Germany?

Although one might argue that because a "5" is a failing grade in Germany it cannot be the equivalent of a "D," I would argue that qualitatively, a "5" is a "D."

"5's" are common even among studious students at the Gymnasium. German teachers give "5's" out freely, especially on tests, in a way that American teachers do not give "F's" out to hard-working students on the academic track. A "5" indicates serious deficiencies the way a "D" does.

"5" and "6" cannot be lumped together at the bottom when American counselors "translate" German grades. Only a "6" equals an "F" in terms of missing the boat.