This happens because images are the only signal VLMs have, whereas humans distinguish between eyesight and synthetic images. We are not surprised when we see three-legged chicken in a research data set; our priors are weaker for images. If you "saw" one in real life, you'd probably rub your eyes and discount it too. Try the same experiment on a robot.
Huh? I'd assume it's a mutant, not store a memory of having seen a perfectly normal chicken
You've never seen someone who's missing a finger or has only a half-grown arm or something? Surely you didn't assume your eyes were tricking you?! Or... if you did, I guess you can't answer this question. I'm actually racking my brain for how to logic this out but I'm just going to bank on that it's likely that anyone over 20yo saw an animal with some visible deviation from the norm at some point in their life