The exact meaning of this comparison is obscure. Basically, a comparison is drawn between missing a target and the nature of evil within the universe: just as a target is not set up to be missed, the nature of evil does not arise in the universe. Most often, scholars have assumed that the setting up of the target refers to the creation or the setting up of the universe; thus the Stoic god did not set up the universe such that evil can enter into it. In this interpretation it is important to distinguish moral evil from “cosmic evil.” For the Stoics vice was the only evil, while “cosmic evil” has no φύσις and thus has no independent existence. The term “cosmic evil” is a concession to everyday speech. For the Stoics, diseases, the infirmities of old age, destruction by hurricanes or tornadoes were classified as indifferents, neither good nor bad. In the providential world of the Stoics, nature or the Stoic god creates the universe in the best way, but these “evils” arise incidentally as a byproduct. For example, Aulus Gellius 7.1,9 writes: “Chrysippus says that these (disadventageous things) were created in accordance with nature, but (also) as certain necessary incidental results, which he himself calls κατὰ παρακολούθησιν.” For more on cosmic evil, see L-S I. 332-333. Returning to the comparison, Oldfather in his note to ch. 27 in the Loeb edition nicely summarizes this view: “it is inconceivable that the universe should exist in order that some things may go wrong; hence nothing natural is evil, and nothing that is by nature evil can arise.”
While this is all good Stoic philosophy, nevertheless I think that that Boter in Mnemosyne 45 (1992) 474-81 is right to point out that these assumptions are not explicitly stated in the passage. Instead of a comparison, the “just as” clause in this interpretation reads more as premise, i.e. since the universe was not created so that evil could arise, etc. Rather Boter argues that we can assume that hitting the target signifies attaining the good. Epictetus focuses on the other side and this is the point of the comparison: missing the target is evil. Even though the arrow will hit something, the point is that it has missed the target, i.e. missing the good (target) is evil. Evil exists only in relation to the good and has no independent existence. Thus, there is no target of evil, i.e. one does not intentionally aim to attain evil.