The Real Clear Politics average of polls in the Kentucky governor race, published Oct. 30, showed Democrat Jack Conway leading Republican Matt Bevin 44%-41%. But in the Nov. 3 election Mr. Bevin won 53%-44%. The polls were wrong, election watchers exclaimed, once again. The RCP average of polls just before Kentucky’s 2014 Senate race showed Mitch McConnell ahead of Democrat Alison Grimes, 49%-42%. Mr. McConnell won, 56%-41%.

Kentucky is not the only place where the polls seem to be getting it wrong. The 15 public polls conducted...