I don't run a German website, so I can't give you exact numbers, but in my experience, software can often default to something like 'en-US' in a variety of cases where you might not expect it to.
This is especially common in poorly localized tools. For example, a poorly implemented browser component inside some other tool or phone app might not properly set this value, and instead default to en-US.
Switching to the proposed method might fix the author's problem, but create a bunch of problems for other users in other scenarios.
Not sure about German-speaking countries, but in Francophone countries, everything is in French. Even if the default is English, the user would insist on switching to the French version.
Switching to the proposed method might fix the author's problem, but create a bunch of problems for other users in other scenarios.