Luck, if used this way, has no real meaning. It's no more luck based than getting a 12 or a 13. As GTLO has stated, at least in the science sections, there's no magic or tricks to them. If you study every section thoroughly, you can get 14s and 15s. If you do not, you can still get 14s and 15s depending on your luck and the sections that get tested. But every test taker could say that for every question and every passage and every subject.
Thus we study, because we do not want to leave our scores to luck. With sufficient preparation and comprehension, 14s and 15s can be achieved quite consistently.
Not going to lie, I think the VR section is hot garbage. I said that before I took my practice tests, during the practice tests, and after my test despite receiving a 14. Garbage.