I used to give a similar question on interviews to test critical thinking skills. I thought this particular wording was rather straightforward. I purposefully obfuscated the question on the interview to see how people would handle reading things and picking out the essential points (like what they might have to do when reading a specification).
It is an interesting phenomenon, but only a small percentage of the people asked got this (and usually it was right away), and they also tended to do better on actual programming problems (revering a link list, etc.).
"Vowels have even numbers on the back" may seem like an odd statement, but we need to realize that all statements are like this.
Some may just be in more familiar territory, like "To drink legally, you need to be 21 or above." But when we approach something that is brand new, complicated, or abstract, then our collective instincts are as poor about this as they are on vowels and numbers printed on cards.