PB doesn't allow you to put in decimals, so it's sort of understood that 100% simply means >99.5% or so. There I simply noticed a stupid prediction and decided to note it down; easy predictions are relaxing, and I had just put in scores of Eva predictions.
This essay exists as a large section of my page on predictions markets on
gwern.net: http://www.gwern.net/Prediction%20markets#1001-predictionbook-nights