I think there's something, some X factor, that's hard to spot and account for, which is one reason why things can seem random. You can call it talent or skill or whatever--a way of making the story feel immediate, making it hooky. I thought I was just lucky at first, but part of it was that I deliberately wrote three books with a really strong and different hook, and then I titled the series and the books well and put covers on them that made them more visible. But it was the idea of the first book, the hook, that really sold them. I started thinking it was more than luck when my next series did very well, even though it was quite different. I think "voice" matters a whole lot, and you don't know whether you have an attractive voice (so to speak) until you put the work out there.
I've been a six-figure author from the beginning, mid-six from Year 2 on, but there was no way I was good enough to be a seven-figure author. My first book was my first fiction, and it shows. The book's sold well (about 100K copies in English ebook, 40K more in German), but I think if I'd done some things better, it could have done much better than that. Only about half the people who read it go on to read the next one.
Now, I think I might be getting there as far as the skill level, and indeed, I'll probably hit high six figures this year. What holds me back now is more the marketing aspect (I freely admit I don't do much), which includes not writing directly enough to my most profitable market. (Because I like to do different things.) I've written 23 books now, though, in these five years, more than 2 million words of fiction, and I've worked really hard to improve. That's mostly my goal--just to get better, to write things that delight people, to hit the notes right.
Perhaps saying all that invites potshots, and indeed, not everybody likes what I do. Not at all. Everybody doesn't have to like what you do, though. I don't think I write extremely mass market--Crystal has a word for it, but I can't remember what it is. Something about having very broad appeal. You just need to appeal to enough people to make up a solid audience.