I looked in the source code for witches as I was making a mod and discovered that the witches used the motionX/motionZ of the entity they are attacking to throw potions where the entity will be, and not where it is.

However, upon testing their aim in-game while moving sideways, I found their potions would never hit me. Upon further testing (printing the motionX/motionZ out when throwing a potion), I found that when the witch checks for motionX/motionZ, it is 0 most of the time, even when the target (the player in this case) is moving sideways.

In other words, witches ain't throwin' nothing.