Well presumably the original purpose was something like testing how many people use the correct reasoning, and they assume that those that got it right used the correct reasoning, and those that got it wrong didn't. Of course you can use incorrect reasoning and get lucky, but the test wasn't...
It seems like the 4% is the number of people that get it right when the question is a little different - when you're told there is a letter on one side and a number on the other. Not sure if it's still 4% for the quiz we were given, which didn't state that.
Assuming the answer says AK7, the difference with the 2 is that if there is a vowel on the other side, then the rule has been followed (because 2 is an even number), and if there is not a vowel on the other side then the rule has been followed, because if there's no vowel on a card then it's...
Who said anything about consonants breaking rules? The card could have a cock on one side and a Chinese poem on the other side. If a card hasn't got a vowel on it, then the rule cannot be broken.
No one said there'd be a number on the other side. There could be a drawing of a cock, or there could be a vowel. If it's the latter, the rule is broken, so we have to know what it is. Is it a cock, or is it a vowel.