[matthewbauer]

A 4th option could be:

If Biden defeats the established nominee, he must have an exceptionally well run campaign making him more electable. That is by winning the nomination Clinton has just met expectations but Biden exceeds most expectations.

[ovet]

I think it could also be that Vice Presidents have generally done well in presidential elections when running after their VP term.

http://www.huffingtonpost.com/jonathan-hobratsch/when-vice-p...