Primary runoff elections tend to have lower turnout than their corresponding primary elections, mainly because there are fewer – sometimes a lot fewer – races on the ballot. Fewer candidates are campaigning and working to get the vote out, which means the election has lower visibility. Many of the marquee races on the primary ballot have either been settled, reducing voter enthusiasm. A further dampening effect is caused when supporters of defeated candidates do not return to settle a runoff between two non-preferred candidates.
Because these broader effects tend to reduce turnout, multiple runoffs in one area tend to boost turnout. In other words, multiple runoffs can have additive effects because there are more candidates raising the runoff election’s visibility and creating more enthusiasm among the voters.