Pena Hit Probability

Can someone please explain this to me?

The count is is 0-1 to Pena. The "hit probability" stat says 19%.

Snell throws a ball to even the count at 1-1. Hit probability goes to 18%.

I'm actually fairly comfortable with statistical analysis, but is this just the Snell likelihood of a walk eclipsing the batter getting to a more hitter friendly count?