I'm no expert, but maybe it's helpful to think about it this way...
We can
assign a run value to pitches thrown at certain places in the strike zone (and around it). 25% of all pitches are thrown into the "heart" of the plate -- as defined in the first graphic linked to in my post. Batters will swing at those pitches 75% of the time. When they don't, they are almost always called strikes (I think it's safe to assume they're called strikes all of the time, but maybe not). Because these pitches are thrown right down the heart of the plate, the resulting run values of balls into play when a batter swings at them will tend to be high. That hasn't been the case, however, with Gonzalez yet so far this season.
If you look at the very right of his Swing/Take chart at the first link -- he's taking about 5% more pitches thrown down the heart of the plate than an average MLB hitter (27% of the time vs. 32% of the time). By doing so, he's cost himself 3 expected runs (-3 in the chart).
If you look again at the very right of that chart -- when he does swing at pitches thrown down the heart of the plate, he is just not hitting the ball with any authority (-10 runs compared to what the average hitter to swings at balls in the heart of the strike zone would produce).
No way I'll be able to figure out how to post two different graphics next to each other in a post, but compare this to, say,
Max Muncy.
As you can see on the very right of Muncy's graphic, he too costs himself a few runs by taking so many pitches in the heart of the zone (-5). This passes the smell test to me, as he's known for his very selective approach at the plate. In contrast to Gonzalez, however, look at what happens when Muncy does swing at those pitches: +17 runs, meaning he is making excellent contact on them.
Hope that helps.
