At first glance, it looks like the Tigers should be running away from the Royals instead of chasing them. Detroit has a strong hitting team built around first baseman Miguel Cabrera, designated hitter Victor Martinez and the surprising J.D. Martinez. Kansas City's only above average starter has been outfielder Alex Gordon. Both teams seem to have good starting rotations, but the Tigers starters are more dominant accumulating a lot more strikeouts and allowing fewer home runs. Still, the Tigers trail the Royals by a half game in the AL Central Division
The Tigers lead the league with a .329 on-base percentage .424 slugging and .753 OPS. That puts them at 73 batting runs or 7 wins above average. Meanwhile, the Royals are ninth in the league with a .682 OPS which translates to 18 batting runs or 2 wins below average. So, there are nine batting wins separating the Tigers and Royals.
The Tigers starting staff is second in the league with a 3.46 FIP while the Royals are ninth at 3.97. Using the FanGraphs WAR statistic, the Tigers starting staff is five runs better than the Royals. So, just looking at the two most influential components of the game - hitting and starting pitching - the Tigers are 14 wins better than the Royals.
If you've been following the Tigers all year though, you know their weaknesses. First, their bullpen has struggled posting a 4.13 ERA which ranks 13th in the league. In comparison, the Royals relievers have a 3.53 FIP. Using the WAR statistic, the Royals bullpen is four wins better than the Tigers.
Another area where the Tigers have done poorly is fielding. According to the Defensive Runs Saved statistic, the Tigers defensive has cost them 59 runs which is second worst in the majors. The Royals, on the other hand, have saved their pitchers 32 runs. That's an 89 run or nine-win difference between the two teams.
Finally, the Tigers base running other than speedy outfielder Rajai Davis and second baseman Ian Kinsler has been a problem. They are 3 runs below average according to the FanGraphs Baserunning Runs statistic while the Royals stand at 9 runs above average. So, the Royals are one win better than the Tigers in base running.
Combining relief pitching, fielding and base running, the Royals are 15 wins superior to the Tigers. Thus, the Tigers 14-win advantage based on hitting and starting pitching is completely offset by the "smaller" facets of the game.
So, that's why we've got a pennant race rather than a run away.