The ACT “curve” generates a scaled score (1-36) from your raw score (the number of questions answered correctly). This curve is designed to correct for minor variations in the difficulty of the test. In this way, there is no advantage to getting an easier test and no disadvantage to getting a harder test.
A common myth about the ACT curve is that the average test taker should avoid a particular test month if a large group of strong students will be taking the ACT that month, and instead take the test when a large group of weaker students will take the test. The (incorrect) assumption here is that the curve will push down the average student’s score in the first situation (large group of strong students) and pull it up in the second situation (large group of weak students).
The reality is that the curve only reflects the difficulty of that particular ACT, not the quality of students taking the test. For example, suppose that in a particular month, a large group of strong students take the test. Even if they all get perfect 36s, your score will be the same as it was had they not taken the test. In the same way, a large group of weaker students taking the test will not affect your score.