You are obviously an exception to this. You are clearly a perfectionist (not a knock, this is a good quality for some people). Considering your perfect and near perfect stats.
When my test day was coming up I felt very anxious, to the point I couldn't think clearly. This was because I knew I wasn't ready and as a result would do badly. I postponed it and studied more. Come test day I was still nervous but it was much more manageable and I used it as a nice boost of energy for the 5 hour test.
On second thought, your statement only makes sense if you are only talking about the bio section. I never felt I was ready for it because I wanted a 30, even though I was confident enough to know I would get at least in the low 20s.