That's like asking why Lorde had a more impressive single era than any of Miley's
Is Lorde a bigger overall star than Miley?
Ariana may have peaked higher, but
both Selena and Demi had more successful debut albums ww and in the U.S. I'm very interested to see how Ariana will be doing 4 albums into her career, what her networth will be, how much social media impact she has made. Until then, there's really no comparison.