I do think there is a crucial difference between Ayu and DCT in the sense that there is a world of difference in how the public perceives them as musical artists. Much of the impact of Ayu's music is dependent on her being seen as the center or the core of her musical identity, even if her collaboration with various producers, composers, arrangers, stylists, dancers, and directors is of utmost importance to breathing life into this identity. The core and center of DCT, however, is borne by two people instead: Miwa Yoshida and Masato Nakamura. Even if Miwa Yoshida is clearly more prominent among the two, the musical identity of DCT is tied in with both and not just one or the other. In the media, Miwa Yoshida is not identified as DCT but as Miwa Yoshida. Also, you don't read or hear news about Ayu's band members or dancers in the media but you do about Masato Nakamura. He is a constant, permanent factor in DCT, whereas Ayu does not always use the same collaborators. That is the difference between Ayu and DCT as artists and, I daresay, this has much bearing on the sales of their records, even if technically they could be seen as kin. DCT has always been seen as a group.
Edit: Ayumiko is basically saying the same thing as I am
