I think only some parts of a CAPTCHA are challenging you (data is labeled sufficiently for mistakes to be considered a failed solve), and the other parts are still in the process of being labeled, which you are helping to accomplish, so those have no influence on the immediate outcome.

At least that's how the old "type these two words" CAPTCHAs worked. It was crowdsourced human OCR of whatever text the machine OCR couldn't make sense of. I'm not sure if "find the bus/motorcycle/crosswalk/light" is the same way, but perhaps, and it does seem to offer leeway when there's only a few pixels of that item in the frame.

What’s sad is, I’ve started to predict how dumb other humans are at filling in CAPTCHA’s. Often there’s a piece of motorcycle, stair, traffic light, fire hydrant, or non-bus picture that users will mis-select, and I’ll do what I think the masses do.

I used to choose correctly but being sent through 5 chains of CAPTCHAs is modern hell.

I'm not certain that that's what is happening, but I strongly suspect it. It is if nothing else a useful mental model: switching my strategy from selecting every square that contains foo to selecting every square that I think that Joe Schmoe would select gets me through CAPTCHAs more consistently.

It’s been like that for years, they probably use a subset for extra annotation.