From an expert in electoral fraud, professor Walter Melbane of the University of Michigan. Money quote:
In 2005 some opposition politicians called for a boycott of the election. The surge in turnout in 2009 is widely interpreted as meaning that many who boycotted in 2005 decided to vote in 2009. Hence towns that have high ratios should have lower proportions of the vote for Ahmadinejad (the coefficient should be negative). He then tested this hypothesis using an over-dispersed binomial model, finding that it worked well for most districts. Suspiciously however, whenever this data significantly deviated from his model, it was in Ahmadinejad's favor.