I think it just has something to do with the way our minds work, and that carrots are rather common. I remember some "magic" trick I used to pull on people when I was younger, basically you string people along through a long series of instructions or questions, they get nervous and are concentrating on what they think you might ask them to do next, and then you ask them to write down the first 2 digit number composed of odd numbers that they can think of. I think people almost overwhelmingly always said 35 (or 37). I don't really know why we think in such a way, but I have a feeling this plays on the same principle.