Vegas has odds of -138 for the Royals to win tonight, equating to a 58% chance of winning the game.
Historically speaking, home teams down 3-2 in the World Series win 70% of Game 6’s (23-10).
The Royals are effectively a +3 WAA (.519) team with Yordano Ventura starting, an estimate made by calculating Wins Above Average for the entire Royals, less their starting pitching. The Giants are +3.8 WAA (.523) team with Jake Peavy starting by the same estimate. The home team has a built in 54% advantage in baseball. Given this, using Bill James Log5 formula, this means the Royals are 53.6% to win Game 6.
There is some arbitrage in reality vs. Vegas model, which has priced the Royals higher than the calculated 53.6%. The Vegas lines may be over counting the Royals home field advantage and the historical averages. Because the historical average is such a small sample with varying qualities of teams, it’s a no-brainer to bet on the Giants if someone gives you +128 odds. The real money line should be closer to +115.
How often has Vegas been wrong on the Royals this year with a -138 line? In 2014, they had 45 games where the line was at or more favorable than -138. They won 58% of these games, which is exactly in line with what a -138 line should generate. They appear to have underperformed in games with a line more favorable than -160, but the sample size is too small to understand if Vegas misjudged the odds in that case.