US presidential odds percentages will total over 100% for most of the race. The extra percentage over 100% is called the overround. In prediction markets, the overround is a measure of uncertainty.
If the market is certain of the odds of each outcome, then there’s only one price that users can trade at. A user wouldn’t be able to buy a Republican sweep of Congress trade at 75 cents if the market was certain at 73 cents.
Elections are never that certain until the votes are counted. In the lead up, users who want to buy a Republican sweep at 71 cents, 73 cents, or 75 cents may all find sellers. That range of prices comes from a range of opinions about how likely different outcomes are.
That price range also leads to overrounds that can reach as high as 10%.