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In general, the data from these two sources is very consistent, though not exactly the same. For Zone%, FanGraphs' PITCHf/x data records about 3-4% more pitches in the strike zone, a difference that holds up over time and for most players. But not Sanchez. His PITCH f/x Zone% was 51.6%, almost 10% higher than BIS reported. With a 2014 the league average for reliever Zone% of 48.5%, Sanchez ranked 51st of 209, or at the 75th percentile. This would indicate he was indeed filling up the strike zone.
Keep reaching.