Typically on any test, you're shown what questions were answered incorrectly. This gives the person a direction to look in learning something when a mistake is made.
On the certification tests, incorrect answers are not pointed out, so no direction is given on where to improve your knowledge in weak areas. While I personally haven't gotten better than 16/20 on the additional guru test, knowing where I'm wrong would certainly help me know where to look and learn new information.
snarked has a post where he alleges one of the correct responses may be incorrect, or ambiguous.
Perhaps test results could show the errors?
The last question that I found that I had wrong was #17 (in the additional Guru test). Unofficaly saying there is a way to check answer by answer.
Working on something like this, just working out the logistics.