Prices should normally add up to $1 (100 percent probability) since there can never be more than one winning outcome in any market. But since the price assigned to each potential outcome is a reflection only of the most recent trade in that contract, the sum of all prices can be higher or lower than $1, especially if prices have been changing rapidly.

It is also possible that the sum of all contracts is less than $1 because traders believe that none of the contracts will resolve to ‘Yes’.