Predicting turnout can be tricky. There's a certain amount of guessing involved.
Mitt Romney "knew" that you couldn't rely on 2008 turnout data to predict the 2012 election results. In the end, though, it turned out that Democrat turnout was higher than he expected and it cost him bigtime. The expected drop in Democrat votes didn't happen that time around.
If you recall, Romney also threw development of a snazzy new GOTV mobile app to a bunch of Dem operatives. Of course, the app did not work on Election Day.