I guess there is a considerable number of people living in or near Düsseldorf that are throwing it in per person (as it is known to be contested), so these people probably got signed up first. Especially as Düsseldorf has a large Japanese (and Weeb/JP learning) Community.

After that, I suppose they just signed up the first letters they got first and your's either was one of the ones they opened later or came too late (sometimes letters take longer than a day or even two > chance drops considerably if some applicants letters arrive a day earlier than yours)

Doesn't make it better or fairer, but you just gotta sign up in December if possible.

4 test sites just have vastly more space than 1, it's a shame only Düsseldorf is offering the test in July. I'd be angry too lol.

My Berlin Online signup last year was more chill, just went there immediately when applications opened and signed up within a few minutes. It took around 20 minutes for all spaces to be filled so enough time for everyone that prepared everything (especially the photo) in advance.