I just think how Prism was promoted like crazy and Yeezus had the guerilla campaign going and Yeezus still outsold it even with how alienating it was. I know Kanye's huge and all, but it's not like Katy Perry's super small haha.
KP lives and dies on sales of singles, like most pop acts. Nothing new really. Miley is wildly popular, moreso than anyone right now really. Tons of market for album. Two number one singles. Sold less than this.