A levels are all about luckWatch
It's about averaging, over the course of 2 years the students whose combined work ethic + ability tend to equal the best marks, it's very rarely fluke. The luck element is very low when looking at final qualification grades.
It's down to hard work. If you're someone like myself, who has extra time due to poor memory, it's even harder. But I have good exam technique and persuasive writing skills which are hugely beneficial in regards to the success in my subjects. Just memorising dates and stats is difficult.
If I can do decent, anyone can. Just gotta put the effort in, for some it's more than others.
There might be a small amount of other luck, but if you're sufficiently well-prepared and have no weak spots you should hardly be affected by it as if you find it hard, everyone will probably find it hard and the boundaries will reflect that.