When I was a student pilot I had to fly a night cross-country flight with my instructor. I carefully planned the route and filled out my navlog. We took off in a Cessna 152 and proceeded to the destination, Carol County Airport in Maryland. About halfway through the flight, I was able to see the rotating beacon in the distance and I continued towards it.
I was 100% sure I had found my destination. I descended towards the airport and entered the pattern. As I landed I noticed that the runway number was wrong! This meant that the facility directory must not have been up to date. I taxied off the runway and then my instructor gave me the news. I had landed at York airport, in Pennsylvania. These two airports are 20 miles apart!
If I had not been 100% sure I would still have been evaluating the situation as I flew. That is why I will never tell you that I am more than 99% sure. This is a safeguard to ensure that I keep thinking and taking in new information to find the truth.
A huge 1% difference
My choice to be 100% sure meant that the new information (wrong runway number) was immediately treated as wrong! But if you take the 99% sure attitude, then you will treat new information as the truth and constantly reevaluate what you are seeing. If I had been 99% sure then I would not have put down my map and navlog. Instead, I would have seen rivers and cities and roads all in the wrong place and figured out where I really was.