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Fielders don't start from the same place all the time. There are defensive shifts put in place, or outfielders play deep or shallow depending on the situation. So a hit to one area of the field in one inning might be a hit where in another inning it would be an out. Hard batted balls right at fielders are outs, but soft bloopers that are placed juuust right are hits.
Defense is a nightmare to predict or quantify statistically.
~G