Interestingly, with Joe Biden in the range (10%, 13.5%) to win the nomination, and (7.7%, 8.3%) to win the Presidency, the lowest his electability could be is

    7.7% / 13.5% = 57%

and the highest it could be is

    8.3% / 10.0% = 83%

so the market seems to be pricing a probability in the range of (57%, 83%) for Biden to be elected president if he won the nomination - compared to Hillary Clinton's range of (56.9%, 57.7%)

I can think of a few explanations -

1. Biden really is a lot more electable than Hillary Clinton

2. Both candidates have electability at the low end of their range (around 57%).

3. The market is wrong, i.e. they are systematically underrating Biden's chance of winning the nomination (and overrating Clinton's) or overrating Biden's chance of winning the election (and underrating Clinton's) or both.

There is no arbitrage, but if you believe 3, there might be a good profit to be made in expectation by backing Biden to win the election, but Clinton to win the presidency (you wouldn't hold it through to 2016, but take off the bet as soon as the odds come back to something that looks more plausible).

I haven't done the analysis to see if it's still worth it after trading costs, but maybe someone else wants to.