Quantity, presumably. Procreate itself ist eben cheaper (or at least it was when I bought it) so the model must be working for them. And I'm the perfect example why: I'm not an artist, I barely draw, but I bought Procreate. Why? Because it's a really cool tech demo for that expensive new iPad Pro and at €10 one-time it was extremely easy to justify, even if I've only played with it a bit. I imagine a lot of people fall in the same category and bought it even if they're not the typical market for digital art apps.