Rihanna and Katy Perry, for example, can't sell huge numbers of CDs (as opposed to single downloads) b/c of their young fan bases.

Adele appeals to a much broader audience ranging from teens to senior citizens. The older the demographic, the less likely they are to download music (hence, CD sales) and the longer they are likely to discover music (hence, that Adele's CD is still selling).

There are more the 300 million people in the United States, so her stateside sales numbers are totally in the conceivable range.