First of all, the "100" milion is crediited to communism overall, not USSR
The claim usually comes in part from a shitpiece called The Black Book of Communism, a work of sensationalist and apparently profascist tripe originally slapped together by professing ‘leftists’ to promote the status quo. The statistic was made using loose qualifications, such as millions of nonexistent Chinese babies being considered ‘victims’, relinquished households probably being considered ‘deaths’ (partially due to bureaucratic technicalities, most likely), and some automobile accidents being considered the fault of socialism. Many of the subtotals are not widely accepted by historians. The statistic also includes many socialists, such as Soviet soldiers and civilians who, of course, learnt about socialism in either schools or on their own. Add this to the fact that most of the victims weren’t straight white cismen either, and normally the statistic would become almost worthless for white anticommunists, which is why they don’t care about millions of Africans, Asians, indigenous Americans, or aboriginal Australians dying. Here is what two of the authors said of the work:
