Well yes of course it's normal! All depending on how careful at handling you are. We are humans after all!
really though, getting a few scratches here and there are normal along the way. Knees bumping, case bumping the wall, moving case on a hard surface, dropping things, list is endless. All our minor unconscious acts contribute to invisible scratches that are visible after much accumulation. But you seem to be a careful being, I honestly and personally wouldn't worry about minor scratches. Most times they're hardly noticeable unless examining up close, and it is only you that's making yourself unsatisfied by those scratches. I'm sure other people wouldn't even notice minor scratches. Unless it's a major scratch, of course, but the process of getting a major scratch is difficult and probably even requires effort.
And if the case sits securely on a throne, then there wouldn't be any natural processes for scratches unless caused by you. Possibly if dust gathered over a long time, then it'd be more prone to scratches. As long as you handle your computer with care, there shouldn't be visible scratches. It's what caused the scratches I'd worry about damaging your PC. Even if it does get scratched, you can always paint it! Although it's a tedious task.
Don't worry, and always treat your PC with care
Minor scratches are perfectly normal, and it's always fixable if it gets too bad. Best wishes!