

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.
Kinda… Slightly more helpful, but almost as vague. I’m advising against opting for solutions that are technically correct but would be more difficult for the average person to get right most of the time.
The OP’s CAPTCHA as a case in point, it’s frustrating for them because they’re ostensibly asked to enter the characters that they see but there are several and the length of the string of characters is not known and some characters are hard to read and depending on how you interpret it you could be being asked to enter all these characters or you could look at them and say there’s a background set and a foreground set in which case, which one is the correct one? That’s at least 3 different ways to do it and that’s assuming that what appears to us a representation of depth is indeed intended to be the basis of separation for 2 sets of characters and not some other arbitrary categorisation or no categorisation. Sounds complicated and ambiguous. Except, it’s much harder to read the background set, and the idea that there would even be some other way of categorising, if it occurs to anyone at all would be impossible to work out since if it’s there, it’s not discernible. The easiest way is to just read the letters that aren’t partially covered up and also smaller than the more obvious, easier to read, not occluded characters and disregard the ones behind it. What’s easiest to do also most of the time turns out to have been the hidden instruction for what you were meant to do.
There’s no explicit instruction to do this, it’s wishy washy and hard to abstract for different CAPTCHAs which is why this advice doesn’t look a whole lot better than “just guess right” but in a way that’s kind of part of why they still have some effectiveness, they’re unspoken rules that humans Intuit. Where some of us, like me before kinda “getting it”, go wrong, is in overthinking and over analysing it. “but what if they mean this? I mean technically it could…” If you’re thinking like that, odds are you’re barking up the wrong tree and the solution is way less sophisticated.