Mirroring movement isn't movement that's hard to hit; in fact, it's generally a lot easier to kill people who try to mirror exclusively. however, you want to mirror somebody's movement to make it easier for you to hit your own shots. It's situational. movement that makes it hard for the other scout to meatshot you is sort of the opposite of mirroring, where you just make sure they're in the middle of your screen.
edit: this is obviously all situational, but it's good when learning to try to follow a guideline and see what works for you