The items one is supposed to select often overlap the grids, so it becomes a kind of Keynesian Beauty Contest[1] at that point; I assume they validate based on how much in alignment you are with previous answers, so it becomes a problem of “What nearby grids would a person reasonably select when there’s overlap”, or, you’re tasked at selecting <some_item> and you see <some_item> in the distant background of the image you’re supposed to classify and you need to determine “How visible would <some_item> need to be for a reasonable person to classify <some_item> as being in this image”.
On top of this, when you’re clicking through image after image after image, the additional frustrating thing is that you’re helping train their algorithms; you’re doing work because their service isn’t smart enough to know you’re human, or, you’re seen as a marginal customer they can piss off by forcing you to work for them for free.
It’s a frustrating experience when it fails, and my current strategy is to leave the website when the ‘I’m not a robot’ checkbox fails.