According to the Jewish Virtual Encyclopedia:
Five million is frequently cited as the number of non-Jews killed by the Nazis. The figure is inaccurate and was apparently an invention of famed Nazi-hunter Simon Wiesenthal. According to historian Deborah Lipstadt, he began to refer to “eleven million victims” of the Holocaust, six million Jews and five million non-Jews in the 1970s. Wiesenthal later admitted making up the figure to promote interest in the Holocaust among non-Jews. Lipstadt, says “he chose five million because it was almost, but not quite, as large as six million.”
Oy vey!