If I'm supposed to select all squares "containing a motorcycle", and one square contains just a couple pixels of the edge of it, would other people select that square or not? I would, since it contains pixels of a motorcycle, but apparently, again, most people disagree with me, because I'll get it wrong if I do that.
It just drives me crazy having to choose what I think other people would choose, rather than choosing what I believe is objectively "correct". Maybe you could argue that's part of the test, to know what other humans would choose. But I don't think that was the original intent/design. I'm constantly second guessing myself on these types of captchas, and constantly getting them wrong.