It's pretty simple math. When you take a strike or swing and miss it doesn't get counted in the denominator on a 3-1 count but it does for a 3-2 count.
Possible outcomes 3-1 = ball in play, walk, advance to a 3-2 count (which doesn't get counted in the batting average)
Possible outcomes 3-2 = ball in play, walk, strikeout