The Difference Between Forecasters and Handicappers

The following post for members is by G. Elliot Morris of The Crosstab.

Prominent midterms handicappers and oddsmakers currently put the chance that Democrats take the House at about 65% on average, while quantitative forecasters (like myself) put the odds between 75 and 80%. The difference between 75 and 65 may seem trivial, but if you think about the odds of Republican House control the divergence is stark: a 1-in-4 chance that Republicans hold the House is about 150% less likely than a 1-in-3 chance. What explains these differences?