I did some experimenting on this and this would seem to be a bug.
Throwing the ball perfectly straight causes the ball to be thrown to the right.
It seems the game is indecisive on what classifies as 0, the side of the screen or the side of the mouse hitzone thing. This means to get a perfect throw, you need to offset your mouse the distance between the side of the screen and the hitzone when moving the mouse up. (See the green markers on Image #2)
Because the ball angles to the right, my assumption is that the bottom hitzone zeroes at the left of the screen, and the top zone on the left of the hitzone. This leads to a constant offset of that left gap.