I debated this one with Lars for a while. Part of me wanted to keep rewards separate. Every difficulty of each level is different, and me as the egotistical level designer wants everyone to play every thing I made, dammit!
But of course, our reasonable side remembers games that forced us to play levels that were far too easy just to get some reward, and that's annoying and tiresome. We don't want to waste your time, so if you can beat a level on "Advanced", then we're not going to make you play it on "Normal" just for some other reward. So yes, it is behaving as designed.