Acho que encontrei a resposta. Nessa notícia sobre a Cyber Monday, eles explicam qual metodologia utilizam (basicamente, usam o Adobe Analytics, parte do Adobe Analytics Cloud, para fazer a estimativa):
"Methodology
Adobe leverages Adobe Sensei, Adobe’s artificial intelligence and machine learning framework, to identify retail insights from trillions of data points that flow through Adobe Analytics, part of Adobe Analytics Cloud in Adobe Experience Cloud. Adobe’s retail report, the most comprehensive set of insights and predictions of its kind in the industry, is based on an analysis of one trillion visits to over 4,500 retail sites and 55 million SKUs. $7.50 of every $10 spent by consumers online with the top 500 U.S. retailers go through Adobe Experience Cloud.* Adobe Analytics measures 80 percent of online transactions at the largest 100 U.S. web retailers."
Adobe Data Shows Cyber Monday Is Largest Online Sales Day in History with $6.59 Billion | Adobe Newsroom
