The reason they add to over 100% is due to the 10% fee on profits which limits arbitrage. Specifically, all profits have a fee assessed but it doesn't account for money lost on associated contracts so if you buy Biden winning the election for $0.90 and buy Trump losing for $0.10 ---> if Biden wins, that contract resolves to $1 but you only receive $.99 due to the fee. Meanwhile you have lost $0.9 on your other contract. This effect is most noticeable in markets with multiple contracts (e.g., who will be named secretary of state).
This is a reason things will get slightly out of line but not to this degree. You can (and I did) buy all the NOs in the Predictit electoral college market and get a guaranteed immediate return.
If you can buy n NOs for less than (n-1-0.1) on predictit you'll make money net of fees.
The bigger issue is trader limits. I could only take $50 worth of arbitrage because I can only own $850 worth of shares in each bracket. They also limit markets to 5k traders (which most markets have hit at this point) so other arbitragers can't come in and correct the prices.
