As others have pointed out, it’s probably the foreground characters. They’re easier to read and less ambiguous from occlusion by other characters.

In general I find you can resolve technical ambiguities or possible loopholes to instructions in these things by asking yourself “what would most people do, especially if not really thinking about it much?” That’s particularly helpful for situations where you have to select all the tiles with x object in them. Often you’ll see that technically there’s a little bit of the object in squares other than the most obvious ones that everyone would have selected and you ask yourself “does that count? Technically a little bit of it’s in this square” but if you just pretend you didn’t notice that and only go for the most dead obvious squares you end up passing. Once I realised this the number of times I failed CAPTCHAs significantly reduced. For some reason the only ones that continued to be a problem were the click a checkbox ones that seemingly analyse your mouse movement because somehow I apparently move like a robot.