Yes, it's true that the contest always attracts more than its fair share of self-portraits, advertisements (perhaps one of the advantages of the contest is that it makes such pictures easy to find and report!), baby photos etc., but it is particularly noticeable in the August contest that some subjects attract more irrelevant entries than others:-
Night
Easily understood subject. Nearly all the pictures are night pictures.
Human Constructions
Slightly more complicated subject, but most of the pictures are still relevant. I have noticed a few baby pictures. I suppose one could argue that a baby is in some sense a "human construction".....
Vertical
Description a bit more complicated. Perhaps about 50% of pictures are relevant.
Natural Frame
Most complicated concept and description. It is sometimes necessary to press the "Get more photos" button several times before one finds a single photograph that is relevant to the topic.