:::::Re: "If it is "generally accepted" that the USSR had no more than 15 million, then how can Rummel have used 60 million if he based his estimates on "existing secondary sources"." Because before the archival revolution in 1990s much more data become available for the USSR. Scholars re-considered their early high estimates, because they have now more or less reliable numbers (that, btw, can be cross-validated by comparison different data sets, central and local archives, etc) However, Rummel (who was active mostly before this date) refuses to reconsider his early theories (although even such a hawk as Conquest partially reconsidered his earlier views). 
 :::::Re sources. Below are two quotes taken from the review on the Black Book. The authors of the BB cannot be blamed in attempts to understate the scale of Communist crimes. 
 ::::::"Practiced eyes will notice a remarkable fact about Werth's history: he revises most earlier estimates considerably downward. Robert Conquest's The Great Terror had concluded with an estimate of twenty million deaths resulting from Stalin's rule (the Conquest's earlier estimate- PS), including the famine; Werth gives us considerably fewer. He is concerned, fortunately, neither to minimize nor to maximize numbers, but to accurately determine what happened." (Ronald Aronson. Communism's Posthumous Trial. History and Theory, Vol. 42, No. 2 (May, 2003), pp. 222-245) 
 ::::::"Nicolas Werth's and Jean-Louis Mar- golin's contributions on the Soviet Union and Asia, respectively, largely shy away from ideo- logical pronouncements and instead relay ar- chival and eyewitness accounts about the depths of terror, repression, and mass murder in these regions. Werth's and Margolin's rejec- tion of Courtois's tone and argument has also led them to denounce publicly the introduction and Courtois's grand total of a hundred million deaths"(Shane J. Maddock The Journal of American History, Vol. 88, No. 3 (Dec., 2001), p. 1156) 
 ::::::