She really had a spectacular voice. I loved her 'My Love Is Your Love' album where she finally had some decent writing to wrap that voice around - not just soppy ballads or dance pop stuff, some real soul tracks. Here's my favorite, a brilliant Stevie Wonder cover:
I don't know, I'm not a music expert, but I do know that those figures are based on sales, so more props to her. Lots of people liked her music, as for me, I always considered her the best female voice of all time.