How Much of Your ACT Score is Based on Luck?

I’ve taken the ACT three times now, and though I do not plan on taking it again, something has been bothering me. How much are my scores affected by when I take the test? By this I mean, how important is it to get handed an easy test? At this point, I’m certain that every test is different in difficulty level; as an example, I got three different science scores, a 31, a 35, and a 33, in that order. My other scores also fluctuated, though math and writing did less so, while reading and language arts also varied greatly. Have any of you had a similar experience? Did any of you get lucky and roll a test that was easy across all the subjects? How much do scores vary based solely on how easy your particular test was?