Some pitchers have trouble delivering the same pitch, but with variations in accuracy when facing left-handed batters Vs right-handed batters, and the other way around.
In other words, lets say a right-handed pitcher is really good at hitting the down-n-away pitch to a right-handed batter, oh, with 85% effectiveness. Now I'm not mixing in the considerations here, nibblers just off the black and such. Only true strikes - four seams with deliberate location.
In every single case, I've had right-handed pitchers who were dead on with 85%-90% with locations for RH batters, but, for the exact same location over the plate, they were only70%-80% when facing a lefty. The same to a lesser degree with left-handed pitchers. Why this lesser degree, I could never figure out.
So, regardless if you pitch or if you coach pitchers, this is something worth making a note on.
It would be helpful to bullpen yourself and get a number down of what's your variation when tossing the same pitch, same location on the plate, but to different batters - RH or LH.
I know this picture below may be a bit confusing, but the boxes are located on either side of the plate to signify how accurate are you for the four basic locations of your four seam fastball.